Because when you make a single catapult you can consider adding 1 or 2 users for the catapults, but maybe in a future you think its better to have 3, or its better to move the user a bit further from the catapult, or rotate the user, this way you only have a pivot for place them more accurate and you make the animations based on the pivot wich is easier and safe, maybe in a future modifying the dae file of the catapult could bring some bugs like when you import a skeleton to blender and change some bone they return to the original position and deform everything.