I believe that the birth certificate is real, as was the short form released years ago. As to why President Obama didn’t release it earlier, I think it provided him with a useful distraction from real issues as well as a complaint against his political enemies. As long as people are obssessing about the birth certificate, they are not raising issues regarding his presidency and policies. Also, since most Americans (correctly) believed before this that President Obama was born in this country, the accusations of the “birthers” was a great way to discredit much of the President’s Tea Party opposition. Many Americans who might otherwise have agreed with Tea Party issues were put off by the “birther” claims, which got more publicity than most Tea Party issues because they were so sensational.
As to why the President released the long form now: I’m not sure, maybe he was afraid Donald Trump could do some real political damage, and wanted to forestall that. Or maybe, like he said, he felt that the rumors were distracting from the political goals he’d like to accomplish.