It provides an alternative UI environment built and optimized for gaming. It has a separate windows manager, a complete ui, and a set of menus to simplify customization of whatever is needed for gaming and power saving.
And quick access to steam store.
It is extremely convenient if you like a console-like experience, but, if you are a tinker gamer, it has anyway a lot of nice additional features.
It is inconvenient as general purpose desktop os, because on update you basically lose packages not installed as flatpack
yes, it doesn't run plasma when it's in big picture, it runs it in https://github.com/ValveSoftware/gamescope along with other tweaks, so it's lower overhead and game windows tend to behave better
it also handles updates to os as well as to steam so you don't ever end up with an update that breaks steam, they're always in sync
And it is somehow moddable, like people created plugins for the UI.
I hope someone ends up adding alternative stores directly there and not just steam. But in any case you can install the respective apps and so on.
SteamOS also uses an immutable filesystem and the system updates as a whole. Because of that, there is no risk of something updating separately and breaking compatibility.
It's fairly common for things to update on regular linux distros and break e.g. anticheat support in Proton or some other thing.
Another thing SteamOS does, at least on the Steam Desk, is actually using two partitions. The updates are always installed to the inactive one, so there's always one image that's known to work. Even if an update fails, the device will simply boot into the intact OS image. Regular distros usually don't have much in terms of fail-safes, so if things break, they have to be fixed manually.
Basically, SteamOS is trying to be as reliable and "hands-off" of an OS as possible to provide best console-like experience.
Mostly just Valve specific software implements to make the experience better. SteamOS has a really good suspend/resume sleep feature where you can just power off the Deck during a game like any other console, then when you hit the power button again it just lights back up to where you were in the game.
The Steam deck is very quick though. I just paused Like a Dragon Gaiden and it took about 2 seconds to go to sleep, left it sitting on the table for an hour or so while I did some errands. Picked it back up and hit thepower button and I was back on the pause menu in about another 2 seconds.
Steam Deck "sleep" is more like locking your phone than it is like putting a Windows PC to sleep