Even if you can't feel pain yourself, you must know from observation. It's quite psychological, but there's a big physical factor too, like the number of nerve endings and the level of neurotransmitters in the body. Some people can't take much more than a few blows. Other people can walk across a high-voltage electric floor that cooks them from the feet up and carry on going. Some people can tread on a mine and carry on running on their bleeding stump. I've seen it happen.

Anyway, I think the monsters count as a 'physical tolerance experiment'. We fight the monsters, they collect data about our fighting. I don't think just driving us mad is the purpose. There's easier ways to do that.