Sometimes the proper tool for destruction is a giant ball of stone flung through the air from a trebuchet. Sometimes the proper tool for destruction is a bolt powerful enough to split a man in two. A ballista can do both. While the stones it fires aren’t as large or powerful as the stone boulders flung from a trebuchet, it is much more accurate, allowing you to target the specific door or wall to destroy. Their bolts, on the other hand, provide reliable defense against intruders or an avatar summoned by enemies. Their weakness is their immobility, they cannot be easily moved once placed and finding the correct range and angle requires time and patience. With repeated use, it is almost certain that some or all of the pieces of the trebuchet will degrade and need to be replaced.
Seems like it worth its bothersome construction. If your idea of defense uses that.
I do observe that whoever wrote that forgot to change the last sentence when pasting it from treb. Unless a ballista is considered a type of trebuchet. But I am more inclined to consider that both trebuchets and ballistas are types of catapult, as catapulting is what both do.