The Makedonian phalanx -- why such depth?

[Paralus]

"The Macedonian Phalanx has been deemed irresistable from the very nature of its construction. Each soldier, completely armed, when he takes close order, on the point of engaging, occupies only the space of two cubits. The length of the pikes..."

(my emphasis)

The very reason Alexander will have wanted those three sarisae at the front of the mixed phalanx!

These would tend to support my view that the Phalanx 'closed up' into 'Half-files' just before combat/charge distance. D

When "he takes close order" does not explain how though. The phalanx certainly adopted close order for combat. Circumstances, though, will have didctated just when a phalanx "closed up".

I'm intrigued how the Macedonian phalanx, in close order and engaged with Porus' infantry and elephants, adopted synaspismos (Arr 5.17):

Alexander himself surrounded the whole line with his cavalry, and gave the signal that the infantry should link their shields together so as to form a very densely closed body, and thus advance in phalanx.

From memory Curtius has them charge through first and then become involved in a bloody battle. How did men insinuate themselves in between engaged and confused files? Did the phalanx withdraw, lock shields and then advance? If so, what of those files decimated by the infantry/elephant battle?