Why does virgil compel dante to learn by doing and breaking off a twig?

While traveling through the Second Ring, Dante and Virgil come across a place with many dark trees, but no actual people. They can hear voices screaming in pain, but they can't see them. Virgil tells Dante to break off a twig from one of the trees, after which the tree cries in pain and starts bleeding. Virgil then explains that all of these trees are souls of people who've hurt themselves during life, those who have willingly discarded their own bodies and committed suicide. Thus whenever the tree is hurt (a branch broken off), they feel pain as if they were dismembered.