Oooooh Ben. Ben Ben Benny Ben Ben. Ben-sterama. The Ben-meister. Bennnnnn. What more to say about Ben? Well, he's crazy. At least after he goes through the Changing, he is. Whatever he saw during that time screwed up his brain something awful, so most of our exposure to Ben consists of watching him foam at the mouth, spout threats, and attempt to kill Thomas.
What purpose could this rage-monster serve? Well, his banishment is an example of how rigidly the boys in the Glade adhere to the rules. He tried to kill someone, so he gets banished. End of story. And why is that important for us to understand? It helps set the scene for how dire the situation really is in the Glade, while also showing us how much special treatment Thomas gets. When Thomas breaks a bunch of rules, he is allowed to skate by on technicalities and good favor… no banishment for our hero. This helps set Thomas apart as a sort of chosen one, which paves the way for his path to success.