There's also grey areas with Steam like when you buy a Steam key for a game outside of Steam through places like GreenManGaming and get your reviews discounted or otherwise flagged arbitrarily based on an opaque authenticity heuristic.
It make a lot of sense to discount all these reviews to avoid abuse. A lot of developers would abuse reviews hard otherwise.
If you buy a Humble Bundle, you get a set of Steam keys for the games in the bundle. If Intel/AMD/Nvidia are doing a promotion for a free game with a purchase of their product, they give you Steam keys. Etc.
Other than selling keys they can also be used for marketing. If you for instance have a game with multiplayer, lots of DLCs or IAP then giving away keys for base game make a lot of sense: even if only 1% of people who grab the key gonna play it they can still eventually buy other copy for a friend, etc.