"In order to get 13 million on board, you have to get their publishers as well as some pretty big titles. And how do you do that, when you directly compete with their business? A publisher's job is to bring a developer's game to the public, for a pretty hefty sum. Getting the indie developers, of course, is a lot easier...and that is exactly what Valve did, starting with itself. After the fallout with Vivendi Universal regarding the original Half-Life and the costs for its sequel, Valve decided to give Steam its first shot as more than just a match-making and anti-cheat program. It used Steam as the activation method for HL2, providing it with each boxed copy.
Read on at Bit tech.