A GPU is used for a lot more than just gaming these days. It’s used to render videos, accelerate normal 2D programs (like some terminal emulators), accelerate some websites/webapps (those which use WebGL for eg); also modern DEs like Gnome and KDE also make use of it very heavily, for instance for animations and window transitions. Those smooth animations that you see when you activate the workspace switcher or window overview? That’s your GPU at work there. Are your animations jittery/laggy? That means your setup is less than ideal. Of course, you could ignore all that and just go for a simple DE like XFCE or Mate which is fully CPU-driven, but then the issue of video acceleration still remains (unless you don’t plan on watching HD videos).
Without the right drivers (typically NOT nouveau, unless you’re on a very old card), you may find your overall experience less than ideal. As you can see in their official feature matrix , only the NV40 series card fully supports video acceleration - these are cards which were launched between 2004-2006 - that’s practically ancient in computer terms and I highly doubt your PC uses one of those. Now recent-ish cards do support video acceleration, but you’ll need to extract the firmware blobs from the proprietary drivers (which can be a PITA on normal Debian as it’s a manual process), plus, even after that, the drivers won’t support some features that may be required by normal programs, as you can see from the matrix.
The natural solution of course would be to install the proprietary nVidia drivers, but you do NOT want to do that (unless you’re a desperate gamer) as there’s a high possibility of running into issues like not being about to use Wayland properly, or breaking your system when you update it - just Google “Linux update black screen nVidia” and you’ll see what I mean.
You’ll be avoiding a lot of headache if you just went with AMD; or even just onboard graphics like Intel iGPUs (if your CPU has it) would be a much better option - because in either case, you’ll be using fully capable and stable opensource drivers and you won’t face any issues with that.