The Makedonian phalanx -- why such depth?

[Sean Manning]

It was probably for morale and the ability to stand up to armoured cavalry. Later fights between pike phalanxes did come to a phase where the men were entangled at close range, and the same may have been true in the Hellenistic period, but I doubt the extra ranks would help with pushing. But there is little research on the mechanics of combat between two pike phalanxes (or between a spear phalanx and a pike phalanx). As with most things martial and Hellenistic, not many scholars seem to be interested.

Also, with a depth of 16 you could advance to locked shields (synapsimos, with 1 cubit of width per file) and still have a respectable depth of 8 men.