When you spend a lot of time in a social network or Blog there is a Google cookie or a browser cookie gathering information about how long you spend on a website. This information is used by Google Panda to determine how popular a site is. The longer you spend on a site the chances are that the search algorithms sees it as more popular or more relevant .. so the logic is that the more time you spend on a site - the lower the bounce rate is going to be for that site, the more popular the site will be, the more page views the site has and ultimately the more successful.
It is beneficial for Blog owners to introduce "adding friends" and "commenting" with rewards to higher levels because they know that the more time they can get people to spend on their site, the higher their rankings will be.
This is the real purpose. It is purely technical and aimed at defeating Panda.