I use it for kb+m games where I have the keyboard set up on the controller, I use the scrollwheel on the back heaps when that's faster than whatever that control is bound too normally, and I just use it a lot in desktop mode when I'm setting up non-steam games. I'm not sure what you mean by lifting my finger but I only very rarely use the touchscreen, it's honestly leas important to me personally than the trackpad.

None at all. I think you're confusing immutable with a non-persistent OS like Tails. No files at all wipe at any point. The /home folder is a normal read/write partition that works just like on a non-immutable distro, and that's where Steam installs your games. The system files are in a read-only partition that you can't modify directly. They aren't modified in normal operations, so they don't need to be wiped. The biggest benefit to this is that the system keeps itself in a "known-good state" so it's always ready to play games.