Most biologists and anthrologists (?) believe that our brains got so large so that we could better understand and communicate with each other. This has obvious advantages, especially back when it first began to evolve. The cons began to outway the pros eventually, and our brain size caps off at about 1400cc (cubic centimeters). Other aps probably did not need it and it may have something to do with the fact that we are bipedal. Many late bipedal apes did have relatively large brains, as a matter of fact.