There are several advantages....
1) Much easier to move a ball from one vehicle (truck) to another (tractor).
2) I tow several items with different ball sizes. Each ball is mounted to a separate ball mount. Pull one pin and swap the ball size in 15 seconds.
3) They make several handy devices which use a hitch receiver for mounting, such as a tow hook, cargo carriers, tool racks, and I've even seen a 12v winch with a receiver mount.