Full compatibility with games, I never need to think or worry about wether or not an update will break compatibility with Proton. Something that had happened a couple times with Halo Infinite and others.

Not needing to worry about getting on the bad side of a developer like with Destiny 2.

Easily use stores that are not just Steam to hopefully discourage a monopoly that is currently largely on PC gaming.

If I want to run things like Discord, OBS, or whatever other thing I want running I don't have to jump through hoops.

Finally the ability to use Game Pass games natively.

Windows has become my primary driver on my Steam Deck. Does what I need it to do, play games. Some games even run better under 10LTSC compared to SteamOS.

some games want kernel level rootkits ("anticheat"), some just block their DRM in proton, and if you want to use game pass PC, well, you need MS store.