Maybe a dev can explain.

Playing A Plague's Tale from the prime giveaway and I really like the fact that using a non-native resolution whilst in borderless fullscreen does NOT change the display size of the game. Instead, the image is rendered in the new resolution and scaled back to the monitor size. This is how it should be, no? In a lot of games the borderless fullscreen mode behaves more like windowed mode, where changing the resolution will change the display size. I

t's a very small thing but I quite appreciate it as someone who uses AMD VSR to render at higher resolutions all of the time. I usually have to go into fullscreen which is inconvenient if I am frequently tabbing out or using picture in picture.

Semi on-topic:
Why do people even play borderless or windowed mode?!

Even while playing some casual turn-based game -- I always play fullscreen. Going borderless or windowed completely brakes any immersion. And if there is some stuff that you are doing in the background, like casually following a chatroom, or updating a news tab -- alt+tabbing should work just as fast as minimizing a window.

When I still had a 400mhz potatoe for a PC, alt-tabbing was a death sentence. It would take ~10mins for something to minimise and just as long to get back to it.
But playing borderless is even more resource intensive!

So what upside or reason is there to play borderless!?

It should but depending on game engine it doesn't always work out like that in practise. To further complicate things, in both Windows 10 & 11 Fullscreen Optimisations (enabled by default) tries to merge the advantages of both Exclusive Fullscreen and Borderless Windowed into one. You can see this effect with things like hitting the volume up / down key whilst in-game will bring up a volume overlay even in "fullscreen" mode which wouldn't happen in "pure" Exclusive Fullscreen mode in W7.

AB summed it up, the way overlaying works on Windows 11 makes it most convenient to use borderless if you're multitasking. Even despite having 2 monitors, Windows will always minimize the game window if you're doing something on the other screen.

I don't know the specifics of how intensive borderless is on my system but it's not something I've noticed.

One non-gameplay reason is that some games won't allow taking screenshots in full screen for some reason, but it works in windowed mode.
Another may be the resolution of old games, or of games you don't have the system to play anywhere near full res, but instead of stretching too much you'd rather keep it on a part of the monitor. (PS: May also be handy if the game doesn't support the monitor's aspect ratio, and then there's the question of whether the monitor will preserve aspect ratio in full screen and leave black bars or not, and even if it does, some may prefer parts of their desktop instead.)
Another may be that you want to see if something else pops up, so not actually doing something else while playing but wanting to be able to notice alerts or something of the sort, possibly from programs that may have no way to disable entering some sort of automatic silent mode if they detect full screen apps running.

Soooo....people use multiple screens and you game on one while checking your wank....I mean Stocks on the the other?

;)

Lol, nah honestly I just like having multiple things going on in general, my gen-z brain needs all the stimulation it can get.

Did that in the 90s. Friends would chill at different machines in the same room. Movie on one. Games or Mtv on the others. Now im just glad I dont throwup glancing to fast around the room....damn old age whiplash!!

So same old habits with new tools. It's hard to imagine age getting rid of all that but that seems to be the trend.

I use borderless if i want to read a guide in a web browser outside of the game, but i use fullscreen when i don't need a guide, as it has lower input lag and works better with VRR.

While UWP is restrictive and worse than GFWL, there's one thing i liked about it: When you hover your mouse cursor to the top, you will see rhe game window like you're in windowed mode when you're in fullscreen, giving you both the benefits of windowed and borderless fullscreen.

fur me, borderless is the only way to play Guilty Gear Xrd Rev.2 kinda-fullscreen with Proton.
it hangs on exiting if I run it in true fullscreen.
*shrugs*

If you're asking why more games don't do that, it might be because it's a more recent trick that hadn't caught on yet or it's more complicated to deal with and "not worth dealing with".

Since everyone is saying how they play, I'll just say anything I can't get working in 1080p I usually play windowed, mostly old DOS games but some older games that don't have widescreen options that I can't manually change in the config file or console.