I’m a steam and gog fan.

I use steam for the ability to plug in any controller I want and instantly be able to use it. I can download controller maps from the community on the fly.
I can stream straight to my TV and play in any room with a TV in it with zero effort. I can join my friends games with two clicks. Remote play allows you to play games you don’t even own with your friends. Works on linux. Integrated workshop. Easy modding.

I use gog for older games not on steam. Works on linux. (Heroic at least)

I could go on with steam, but those are the key points stands alone games don’t offer.