Phalanx warfare: Closing of the ranks

[Bryan]

So if ranks three and beyond in a hoplite phalanx didn't use spears to fight the enemy in the clash, besides replacing casualties among the promachoi, I think the big question is, what role did they serve? Why add depth if their spears are useless? Why not just take the extra bodies and extend the line outwards to attempt to encircle the enemy? Or add multiple lines, like the Romans, so if the first phalanx breaks the other can take up the fight?