What did the Romans use the trebuchet for?

The trebuchet is a catapult device used to throw an object. The classic trebuchet was not perfected in Western civilization until the Middle Ages. That would mean that it was not available for the Roman legions to use. However, the Romans did develop the Onager catapult which was a predecessor to the trebuchet. The onager was named for the Persian wild donkey because it had such a "kick" when fired. It consisted of a throwing arm powered by a "bundle of twisted cord" and was used to fire projectiles or smaller bundles of "grapeshot" at an enemy army or encampment. During the Middle Ages the trebuchet was a larger version of the catapult and was used to throw objects at fortified castle walls or fire weapons over the walls. In some cases, even dead, diseased bodies were catapulted over the walls in an early form of biological warfare.