Overview page: https://www.gog.com/en/anniversary
Direct sign up for Galaxy Linux beta: https://www.smartsurvey.co.uk/t/YEF5EV/
Often you don't know or care your game won't run offline until your Internet is down and you just want to play a game and it won't run. Also some kinds of DRM are totally invisible until certain conditions happen (say, you upgrade components in your PC and now the DRM thinks it's a different machine).
Even if without fail you install and test every single game you buy from Steam games for DRM before you actually play them (so as not to exceed the 2h), making sure you do so within 14 days of purchasing, and refund them if they do have such DRM, or for every single game you're considering buying, spend time trawling through support communities upfront to see if they have such DRM, you still won't know for sure if that game has some DRM that will stop you from playing it a later date - maybe the offline stuff only triggers after X days offline, maybe the DRM is doing the whole machine ID stuff and breaks only if you change enough or specific components in your PC, maybe it's just something that cause needless problems running it under Wine, and for all those things maybe nobody that had problems with it wrote about them in whatever support forum forum you checked.
Unreliable workarounds aren't actual solutions.
Meanwhile you can buy from GOG without such hassles or risks that you have when buying games from Steam to make sure they're free of nasty DRM, a totally unecessary massive hassle as Steam could easily list for each game if they gave DRM or not and which kind of DRM they have if they do, but they choose not to.
My point is that with its current stance Valve is at best purposefully supporting anti-consumer stances from some game publishers which would suffer in terms of sales if their bullshit DRM was prominently signalled in the store listing for their games.