The Makedonian phalanx -- why such depth?

[Dan D'Silva]

I've been looking at lurvely illustrations of the Makedonians like this one and something isn't clicking for me. The sarissas were meant to keep the enemy at a distance, and the first five ranks or so could all reach the points of their weapons past the front rank of shields. But what were the ranks behind the fifth or maybe sixth for? If I understand correctly, increased depth in a Classical phalanx was used to increase momentum when shoving forward shield-on-shield but I thought a Makedonian phalanx didn't function this way, thus the longer spears. Am I mistaken? Or did the "extra" ranks provide pushing power in some other way, or strengthen the formation against breaking up?