For themselves! It's also not a great system, lots of legacy. More to the point though, it's not a commercial system. Steam behaves as a distribution platform and licenses the publishing rather than the distribution.
Which is not at all interesting for self-published games (be it indies who want to avoid steam, big publishers with their own systems etc).
Steam supports any game from any publisher, including self-published games. Steam is exactly what you get when you try to solve this problem, because none of the big companies want to be involved, since they see it as competition. Blizzard and EA are the only two big companies I know of that are not on Steam, and they both have direct competitors to it that are vendor-locked.
Which is not at all interesting for self-published games (be it indies who want to avoid steam, big publishers with their own systems etc).