The benefits of likes and friends in forum with a lot of content and members are. In addition to what Aaron stated.
If you value what a member says or opinions, you can view all of his posts that were liked and ones he liked by going to his profile (there is a tab for that.
Befriending someone has its benefits to, if you have your notifications set up... Every time a friend creates a thread or makes a status update you will be notified ( the little bell icon on the top). Which is useful, if you don't want to miss a thread after being away from the forum for a bit.