Gamasutra has a good interview with Linden Lab's Rod Humble, who goes into more detail about why he bought Desura, the digital distribution service. Basically, Iris pretty much called it last week, but it's nice to hear it coming straight from Humble's mouth:
"[We want] to make it the most open, developer- and user-friendly distribution service for all kinds of digital goods, starting out with games and mods and going from there," CEO Rod Humble tells Gamasutra. "For us it's a natural step... We're about user-to-user transactions and empowering people's creativity."
So basically, Linden Lab wants to Steam-ify all user-generated content. That is to say, make it possible for people to buy and sell UGC on an open digital distribution market. Steam is a great service if you're an established developer or a somewhat established indie developer, but kinda restrictive and hard to crack if you're a newbie. And yes, Desura's basically targeting the Steam market: