Because sometimes to make a character stronger, you have to hurt them. Put a character in pain, and you can use that to do any number of things. Motivate, disable, use the characters reaction to it to show insight into them.
For example, for Bug the pain is a weakness Eric has to overcome, a way to put Eric in danger and rack up the tension, and a way to show Eric determination to continue learning magic despite it. The fact that Eric Ashgrove never shies away from using his powers when he needs them, despite the damage it does and the pain it causes him, reveals something about the character in a way that doesn't have to be explicitly stated.
Hey son, wanna' learn how ta' make witch balls?