Valve get nothing when you purchase a steam key from another sales platform. They allow the free use of their drm and content delivery platform to acquire new customers, and keep existing customers locked in.
Re:IGN acquires Humble Bundle
by penguinUzer on Saturday October 14, 2017 @02:25PM
(#15473453)
Interesting. This is the closest source I found with a quick search: https://partner.steamgames.com/doc/features/keys
Phil Fish who created the indie game Fez (which I also purchased) stated Steam & GOG take a 30% cut whereas Humble only took 5%. http://www.pcgamer.com/steam-and-gog-take-30-revenue-cut-sug...
Personally DRM free takes precident. I haven't bought a AAA game through steam since Portal 2. I buy games through GOG to get a DRM version - for example I bought Firewatch from GOG and played it in Linux. DRM free Humble Bundles next. Humble store steam keys as a last resort.
However, I just bought two book bundles over the past month. They are still fantastic. 10-15 books DRM free for about dollar or so each, and a portion of that money I specified to be donated to FSF.
Hopefully post IGN aquisition the book bundles will continue.
PS. I bought Cuphead week of release on GOG to show my support. It may be Windows only but it was DRM free at launch.