Why does the serving player hold up the new ball to show his opponent when the umpire asks for new ?

I mean both players and everyone in the crowd for that matter has just heard the umpire say "New please," so why does the serving player still feel the need to hold up the new ball to his opponent when the player at the other end of the net knows full well that he is serving with new because he's just heard the umpire requesting a change of tennis balls.