KDE

Becauase some people like the idea of a single device that is a pretty good lower resolution gaming handheld, and can also basically be used as a low/mid range laptop/tablet as well.

The DE mode uses a very nice onscreen keyboard which works either with your two thumbs on the Steam Decks touchpads, or, the whole screen is also touch sensitive, and you can do it that way.

Or you can just dock it and use an actual Mouse + Keyboard, and/or a distinct monitor, which can handle DE mode at a higher resolution than the Deck's built in screen.

Also, if you're into modding games or using emulators, you need to go into the desktop mode to tinker with things.

(I was actually quite surprised to find that a 3DS emulator just... worked, perfectly, with the Deck's touchscreen, without me even having to manually configure anything... so i could actually use the main controls for the main screen, and just poke the other screen that is usually some kind of inventory screen, or something like that.)

Or if you want to say... check your email.

Browse the web on a fully functional desktop browser (of your choice), watch youtube, go on lemmy or reddit, buy some stuff off Amazon or w/e.

Draw something or do some photo editing in Krita or GIMP.

Do office work style stuff with LibreOffice.

Use Discord, or record your own gameplay with OBS or something, and then edit and upload a (admittedly low resolution, most likely) video to youtube.

I've actually compiled from source and run Godot on a Steam Deck.

You can literally develop a game on a Steam Deck, and then test it in game mode, on a Steam Deck.