Facebook Connect, OpenID and similar tools offer a great possiblity for websites to more easily allow their users to access. OpenID was an initial concept, but Facebook has the audiance… Twitter, LinkedIn, and other popular features also have similar programs in place.
The benefit for the site is simple; you improve signup rates and gain access to a vast array of integrated features and data points from Facebook profiles to like buttons, status updates and the like. For a user the idea of filling out a whole registration page when they already have accounts is well, silly