Slapstick can be understood as a positive reinforcement for recognizing the incongruity of behaviour which should be causing extreme pain and injury, but which doesn't.
As a thought experiment to illustrate this, choose an example of slapstick humor, and imagine that it caused the form of injury that it rightly should, or which it might well have. It seems that the humor drains right out of the situation.
Slapstick humor reinforces our understanding of what causes pain and injury, and teaches us how to avoid pain and injury.
I don't want to be too absolute about these definitions, however. It is certainly possible for humor to play multiple roles, both internal and external to learning. Slapstick also seems to play a large role in reinforcing our understanding of social positions and the behavior towards and by those in particular societal roles.