I can’t cite a specific source since I was researching the subject for a fantasy novel I was writing at the time, and I’m not even sure the material I was reading was in English, but I remember the author was making a comparison to Roman legionnaires throwing their Pila synchronously to maximise their impact/psychological effect. And it made sense to me since every soldier only had two to carry.

Apparently shooting them in single massive salvos would force their enemies to crowd into one another (they’d have to push someone else into the path of a Pila to avoid one that’s coming at them) which devastated their morale.