People are dumb and like to be told how well they are doing for just playing the game. They are dumb and competitive so they like to have worthless virtual trophies to display. Hooking them on this pointless nonsense is very easy, and once done it works as a sort of "soft DRM" that keeps them leashed to your platform and using a client, in such a way they don't mind it, because they can no longer imagine gaming without them anyway. It is also a way to pad out games with pointless activities so it seems there's more to them and more replayability than there actually is. Basically it's a cheap, easy way to exploit primitive impulses of the consumers against them and make them happy about it.

In general, people have always liked to compare themsevles to others, and will always be motivated to continue in pursuit of a goal, where they otherwise wouldn't. Some of us just like to achieve our own worthwhile, personally defined goals, and others are content with achieving something predetermined. But a need for them, I am not so sure about that. Many people care little for them, but they are simple to impliment, increase the retention rates of at least some players, and have just sort of become a standard and developed slowly over time. You are perhaps one of the fortunate few who is content to engage in a hobby alone for its own enjoyment.
Even though are you from the physical format generation, you have likely done something or known someone who has done something similar in premise to an achievement: be it going for a high score, record time, uncovering an unnecessary secret, sharing your acomplishments and progress with a friend, etc.

This is bait.

Achievements is just an extra metric devs look at to determine which parts of their game needs work. And it becomes easier for them to use that data to design their updates and sequels rather than having them figure it out themselves. It's part of a major problem that encourages rapid deployment of releases rather than spending internal testing.

But they disguise it in a malicious way to exploit human psychology so that their players get dopamine hits when they hit milestones. Unfortunately, it worked too well so now we have a ton of people actively putting more work on devs' plates to implement than to not.

If one is playing games solely for achievements rather than for the story, characters, gameplay, ending, etc. itself before the mid-00s, then I argue they can better obtain their dopamine hit from any other 'achievement'-filled programs like mobile apps, Duolingo, etc. instead.

E-Dick.
But more seriously, it can be a useful metric for the lazy; to see how many people see a game though, what paths they take, what they find. And goal oriented marketing whackjobs who don't understand the idea that time is finite can be shown, "We put too much content in the game, can we reduce scope for next game" and the marketing team will tell the design teams, "no" when asked.

Because the damn bastards won't listen anyway.

Edit: Clarified my statements a little.

I like achievements as an extra feature. I hardly never 100% complete the games though.
Still, if pc games wouldn't come with achievements, I nevertheless play them. During CD/ROM era it didn't even matter to me on PC or consoles.

Some people want to 100% complete games, and they can't if they don't get all the achievements.

It's also said to extend game-life in the form of challenges, or hinting at things in the game that you may have missed and where to find it.

But otherwise it's pointless.

Honestly i originally thought the scored points was a currency you could use to purchase extras/DLC on Xbox Live, which could have been a cool way to encourage DLC and free content if you played enough.

Probably a way for users to brag to other users as they compete for the rarest achievement available. And game developers can also use achievements as a metric to measure the players' interest in the game. The most extreme example is an achievement that you get from completing the tutorial. If the number of people who achieved this is less than 50%, then you know that something is wrong your game. Maybe the tutorial sucks, or idk.

Plus, many achievements tend to have amusing names. So if the thrill of competing with other players isn't what you're after, then you'd probably just chuckle from the notifications every now and then.

In addition to what Breja said, many of them are nothing more than obvious telemetry as a substitute for play-testing. Eg Baldur's Gate EE (which wasn't even originally designed to have achievements):-

"On My Own" = "I left Candlekeep, the only home you've ever known." = You literally get an achievement for leaving the 1-screen, 5 minute long tutorial... "Friendly Faces at the Friendly Arm" = "I talked to someone I was told to talk to anyway". 99% of the rest are the same "Quest Completed" you already get labelled as such in your journal. Same with PlaneScape Torment EE : "Mortuary Gate" = I Escaped the Mortuary. Another "I won against a tutorial" badge. Half the rest are "Story related, cannot be missed" glorified telemetry.

Some people like to believe achievements "invented" unusual / more interesting challenges but the truth was we were already doing that stuff anyway without any need for validation, eg, No-Aug Deus Ex / No-Plasmid Bioshock / Melee-only System Shock / non-lethal runs, "Ghosting" Thief games (no kills, knockouts or detections), Doom on Nightmare but starting every level fresh with just a pistol + 50 ammo, mage / summoner only RPG builds, Speedruns & Glitch Exploitation (Oblivion "paint-brush ladders", permanent enchantment stacking, the "I leapt 10,000ft to kill God" Morrowind Speedrun, climbing the tallest tower in Deus Ex Hong Kong using a "LAM ladder", etc). It used to be called 'imagination'...

Some people like to brag about doing something but even that has long been worthless thanks to SAM (Steam Achievement Manager). I've literally seen people unlock +8,000 achievements in several hundreds games in the space of 12 seconds just by issuing the right Steam API calls. Failing that there's Cheat Engine (Downloads correct .ct table for the game, changes "Gold" to 2,000,000 and "Monsters killed" to 50,000). "Achievement Unlocked : Rich Man : You've earned 2 million!" / "Achievement Unlocked : Slayer : You've killed 50,000 monsters!" Well that didn't take long. (Exactly why proper World Record Speedruns require a verified video (and an audience watching in real-time), not a "badge" that may or may have actually been earned)...

I think my favorites are in Bard's Tale 4, where you get a separate achievement for just starting the game as each class. So you get four different achievements for just starting the game.

I consider most achievements as pseudo content to artificially inflate play time. However there are some games which do it right and make completing them actually fun. Guild Wars 2 comes into mind which has a plethora of achievements and meta achievements (e.g. complete at least 7 out of 10 achievements from the following list) tied to every activity in the game. So if I'm exploring a zone I get one for finding the secret lair, one for defeating a certain boss roaming around, one for completing the corresponding events and so on.

I'm not doing this just to check some boxes, though, but to earn points to progress my account in a meaningful way. Among other things I get premium currency which I otherwise would have to buy via credit card so it not only gives me direction in an otherwise really non-linear MMO but it is really rewarding as well.

It started with Xbox as a way to show your "gamer cred" and then steam and Sony copied it, after realizing kids were really into it
But it's also just a super cheap way for developers to increase the game length, since people will grind the most stupid shit or beat the game 7 times just to unlock one achievement

So a pretty dumb thing all round, you can make your own achievements if you want by imposing your own rules (a Pokemon nuzlocke, a low level clear of an RPG etc) instead of relying on achievements

I didn't see the point of achievements before, until some pointed out the social aspect of it, ie. letting other kids in the school, who are your gaming buddies in Steam or whatever, to know what achievements you have finished. They can see it themselves through the service.

It was also a good point that considering that, it might not make so much sense to enable achievements in GOG games unless your Steam gaming buddies can see them too on Steam.

I earlier thought about it only through my own experience, ie. why would I care of arbitrary goals in a single-player game that don't seem to have any real relevance to the rest of the game (like e.g. finishing subquests in a RPG are normally).

Maybe it is because gaming isn't really a social thing to me for the most part, I don't care to have gaming buddies with which we play the same games together etc. When I was a small kid, it was different, remember e.g. talking about Dungeon Master etc. with other kids in the school.

It is enough achievement to me that I finish some game, and the social aspect of it is that I might mention about it in this forum, if I care enough to report about it. I know you all are like "wow that guy is so cool. he finally finished Blood Omen 2, after all these years...".