Ben is the son of a man who used to work at Manderley. He's emotionally unstable and seems to be mentally challenged. One thing's for sure: he doesn't like Rebecca.
He tells Mrs. de Winter that Rebecca threatened to have him committed to an asylum if he didn't stop peeping in her windows or if he told anyone the things he'd seen her doing. (What exactly had he seen? It must have been pretty bad!)
Ben might be around as a contrast to all of the manipulation and deceit occurring at Manderley. We seem to think he isn't with-it enough to manipulate anyone: does that mean we can trust his version of the story? Or is he covering something up, too?