Per the AP. He did it to quash the lingering "debate" over his citizenship.
The birthers are loons, impervious to evidence and motivated solely by an irrational antipathy toward Obama. I'm sure Obama and his advisors know that, and are only hoping the full birth certificate will forestall mainstream Republican efforts to stoke this particular fire in the 2012 election. However, "mainstream Republicans" are, if not quite an oxymoron, certainly an endangered species. Don't be surprised if the Republican primaries feature presidential hopefuls who continue to bleat this arrant nonsense to their increasingly out-there base.
Like the non-debate between evolution and creationism (aka intelligent design), the "birther debate" is a bogus controversy: the only ones who think there's anything to discuss are those who don't know what the hell they're talking about.