Mostly for tinkering. If I’m setting up an abandonware game or modding a game I prefer to do so in Lutris. It makes it easier to hop back and forth.
I also use desktop mode for a lot of indie games I may only be playing for small amounts of time
Currently, just when I have to use it to install anything that I can’t install via game mode, and when I need to tweak the system or game files, copy roms from my NAS, etc.
I connect mine to work monitors via a dock/hub. I use it to browse the internet as well play some text based web games.
These days manually passing save game files between it and my other PCs.
Sometimes to install stuff like emulators or when needed other launchers.
I mostly use desktop mode for messing about with things like emudeck and creating and forgetting an su password.
I almost exclusively use desktop mode. One of the main reasons I got a steam deck was to have a Linux PC connected to my TV at all times. I watch a lot of sports via pirated streams, and those only really work well in a web browser. A raspberry pi 4 was not quick enough to keep up with it, but the steam deck is more than capable.
Tinkering with settings, web browsing, coding, file management, terminal, DistroBox, YouTube watching, using multiple monitors, installing flatpaks, installing emudeck, installing cryoutilities, compiling code, Qt creator, etc.