To stop a wedge.

[M. Demetrius]

Any infantry advance can be discouraged by caltrops (tribuli), or other ad hoc field traps/pitfalls/"lilies". Not every battle was fought on unprepared ground. Frequently, commanders picked the spot for the battle, and camped so that they could have that land they considered fortuitous.

So a horde of charging infantry, finding themselves suddenly with sharp things sticking through their feet would be in significant distress, and could be much more heavily damaged by missiles.