Yes, unpredictability is the main purpose of a pop bumper. Infinite possibilities of randomness which helps make a machine enjoyable to play.

Along with the aforementioned, I like the strategically placed pop bumper blocking a section of high scoring elements of a game. 

masochist! I was interested as I note even the new WOZ has 3 of them. Avengers has 3 and it just slows evey thing down. Odd things, but I understand their use.