PixelBoy: They are not optional and there is much harm from them just existing.
They are a completely optional feature in games. I'm yet to be harmed by them either. Unless you meant some "games" on steam, e.g. the ones with 1000 achievements, where it might be the main feature. I'm not the target audience, so it didn't occur to me you might mean those.
PixelBoy: Even if we disregard the fact that they are meant as a form of DRM, as stated by Steam's own documentation, there are several downsides from them existing in the game.
Steam drm is separate from achievements as a feature. You can have drm-free build on steam and have achievements too. If you launch the game without steam client, achievements won't upload to the server and your profile. Unless the game checks save file at start and triggers achievement unlock at game launch or something. These technicalities all depend on developer and are per game basis - you can't lump it all together and generalize. Sometimes removing the steam or galaxy api libs is all it takes to get rid of them in such cases.
PixelBoy: They cause unnecessary and immersion-breaking pop-ups to appear in the game. In some games that is not a problem, but in many games those are hard coded in the game, so even the seemingly DRM-free stand-alone version shows them.
I'm all for immersion in games! Like I'm totally insane when it comes to this in story-based games, horrors etc (dark room, headphones, etc - you know the drill). I have my overlay set to not popup ANYTHING, no messages, no achievements, screenshots, etc. I tend to even turn off (or make it almost fully transparent or de-clutter hud in games, if it is possible. You can do that too, no pesky achievement popups - or you can disable them in galaxy client altogether, if you use it.
PixelBoy: What's even worse is that achievements affect the way games are designed. Because there are some achievements that most people will complete, and some that most people won't, in some cases that causes developers to adjust the flow of the game based on how achievements are handled. And that, if anything, is the worst kind of reverse thinking in game design.
I know of no games that I played that would be designed around achievements first and foremost. Maybe we just play different games, or you meant those "games" I mentioned in the first paragraph. Or you mean mobile games, or some freemiums, mmos, mobas or something? It really works the other way around, you add achievements at the late stage of development. They can be used as a statistics tool for developers to see what % of people reached some stages/levels of the game, how many finished it, on what difficulty etc. Definitely less intrusive tool than some hidden trackers, analytics and data miners - aka the user experience plugin.
PixelBoy: I could go into a philosophical discussion about achievements existing mostly outside the game, and not in the game, but I let that pass. What I am going to say is that achievements change the game from being a game on its own, to some bizarre show where people compete on collecting some non-existing trophies, and that is somehow the mark of a better or worse player. What happens in reality is that "the best players" end up speedrunning games with a walkthrough, just trying to gather as much achievements from as many games as possible.
I'm all for these, I made challenges for myself in games, esp. after I got bored of the main course while still enjoying the game and its mechanics. People playing games with walkthroughs because they don't want to or can't afford to put in the effort and resources is a chapter on its own. I find it strange too, but it is what it is - guides are online at one's fingertips and if the want to spoil it for themselves, let them be. At least the very first run, I always go in blind (no guide, walkthrough, youtube vids etc).
However, using cheats just for the sake of getting the achievement is quite sick and I've came across quite a many of cheats, esp. in those 100% achievement groups. But it might be a matter of character, some insecurity or even an underlying mental condition at play there. A chapter of its own.
PixelBoy: So in the end, the achievements don't work as allegedly intended as a mark of a good player, they may to some extent work as a form of DRM as intended, as there are many people asking them to be added to DRM-freely distributed games too.
But the argument of just disregarding achievements can be turned 180 degrees too. What prevents each player from setting his own achievements, like many people do. Reaching one million points in some shooter? Playing through an adventure game without a walkthrough? Complete an FPS game using a dance mat as a controller? Great!
Wouldn't all those be real achievements that mean something to the person who achieves them? How does a pop-up "You have entered 50% of all rooms in the game" give a sense of satisfaction to anyone? Really?
Achievements in DRM-free build can be handed well too, e.g. you have them in main menu as a list of trophies. I know of a couple of games that does it this way. I think Children of morta and some other game I played recently has the ability to turn on ingame popup if one wants it (defaults to off in pretty much any game I played). Second way to implement it in drm-free build I already stated earlier - hooks/triggers in place, but if you launch it without client, nothing really happens, so you can be at peace.
Bottom line here is that one can easily work around achievements, if one can't stand or simply ignore them. If it bothers you to see them in some ingame sub-menu though, then the problems lies elsewhere...
Me personally, I do enjoy them cheevos, because when I look years back on some of them, I see their timestamps and accompanied screenshots sometimes, it reminds me just like photos of what I did then and I remember some of the feels, like when I did some 1CC runs in arcade games on harder/st modes, etc. I didn't forget my Quake 1 run on nightmare in '97 or so, despite having no badge for it, but whatever.
Remember it's just part of the game and just like in life, it's totally up to you what worth you attribute it with, if you still can't work around or ignore them.