purpose was almost certainly to transmit disease. In midieval medical theory the stench of rotting organic material was thought to be a potent cause of disease. And in two of the three written records we have of these events, the victims interpreted the stench resulting from biological aggression as the cause of subsequent disease.
Now how sick would that be if were were able to launch plagued human cadavers over the walls of a "turtling opponent" This was a real method use throught history in warfare. With the creation of Universitys etc... should come the tech of biochemistry warfare. Just imagine your in a long battle, you haven't a clue whats going on but all your villagers are slowing down, starting to cough, turning black in color. Then 1 by one they start to die off. You can't reproduce them fast enough. Soon the lack of resources efferts your battle efforts. Awwww man this could be ingenious. To counter it you would have to have a universtiy also and perhaps develope an cure or something. Just so food for thought