Priming the Pump

Perhaps another guy out there can enlighten me. Men's room etiquette follows many unwritten rule. For example it is not acceptable to have lengthy conversations with your urinal neighbor--stall-side communication is strictly prohibited. Shakes are limited to no more than two. These rules are learned from a young age through frequent washroom use. But there is one such men's room ritual that continues to baffle me. Why do men spit in the urinal while peeing? Is this some archaic tradition remaining in practice by our elders. Or...is this some way to prime the pump? Does spitting somehow get the ball moving when you get older?