It allows selectivity and variety while allowing a larger amount of control over the dating experience.
Serial dating also lets the dater experience several levels of relationship at once.
This decreases boredom while increasing the chances for amusement with a variety of interesting people. If he refers to you as a friend, both in private and in public, he is letting you and everyone else know his options are still open. Casual conversation does nothing to reveal whether or not he is having or has had a sexual relationship with any of these friends, and you are deliberately left wondering what makes you any more special than the others.