Kerethites and Pelethites

1 Chronicles 18:17 - Benaiah son of Jehoiada was over the Kerethites and Pelethites; and David's sons were chief officials at the king's side.

I wonder what the function of the Kerethites and of the Pelethites was. Why were they acknowledged as separate groups like this? What made them worthy of mention? Where did their names come from? Groups of peoples were often named after an ancestor. I'm not sure if that's the case here.

I wonder why a description of their purpose or origin is not given. This would have been obvious to their contemporaries in Israel, but not to us. I wonder if neighboring peoples knew all about them.