The follow function was originally developed in order to facilitate the formation of self-congratulatory backslapping cliques, as it was decided there were not enough of these on the internet. It makes it simple to piggy back onto somebody elses argument in order to demonstrate your most excellent cleverness to all your chums. With just a few clicks you have access to all the flame wars and nit-picking occurring in your chosen group so you can join in the action without having to read all that tedious filler that clogs up the rest of the forum.
Also, by looking at your profile, other people can see your list of followers and followees so they can be in no doubt where your allegiances lie, should they find themselves in the mood for a bit of online argy-bargy.