What was the practicallity of Ballistas in Medieval Warfare?

by Someone4you

And what was there purpose in the battlefield? If I were to guess, they'd be used to dismantle enemy artillery such as catapults, trebuchets, etc.

wjbc

Here is a post addressing how ballista were used by the Romans which might also tell you something about how they were used in medieval warfare -- mainly in a siege, and not against the walls but against the people manning the walls or the defending artillery.