Basically, from the point of view of visitors, people don't want to create yet another username/password they have to remember and hope that the company they signed up with isn't as incompetent as Adobe, Sony, or LinkedIn.

From the standpoint of the company (leaving aside for the moment privacy concerns of the user), if it has users login in with a social ID, it will get access to their profile information from the identity provider.

The downside of using social login is, of course, that many people are rightly concerned with what is being done with their profile data.