I see it as a painless order fulfillment service. If I develop an open-source hardware design and link to the OSH Park project on my blog (or whatever), then others can follow the link and get the PCB made with a minimum of effort. Saves having to download Eagle, create Gerbers from the project files, find a provider that can make them, etc. etc...
To anyone with a little PCB experience, it's probably not an obvious advantage. But in the world of Arduino and kits, it's one less thing a budding hobbyist would have to deal with. Its success is entirely dependent on those contributing though.