Many of those allow you to copy the files freely and move them between computers, even to computers that don’t have Steam. Hell, I played Half-Life on my Linux handheld yesterday, and I didn’t install it through steam. Just copied the files onto the SD card, set up a script to bind the controls, and launched the game.

Now, I’ll concede you must initially sign in to get access to the files, just as you must sign in to download installers on gog.

I use both often though. I play a lot on the aforementioned Linux handheld, and you can port pretty much any DRM-free game to it. Most surprisingly work better using the Steam version than they do the GoG version, though that is likely to be down to the handheld and not anything the stores are doing.