Running BGP4 on a Core Router

Hi folks, I was hearing that typically BGP4 is run on all routers inside a POP, including access routers connecting to customers, border routers connecting other ISPs and core routers connecting to other POPs in the same network. I can understand why BGP4 is run on access and border routers. But running BGP4 on core routers is beyond my understanding. I thought you don't need to run BGP4 on core routers which are considered to be interior nodes. Can someone shed some light on what is the benefit of running BGP4 on the core routers? Thanks, Hansen