How is friction harmful when riding a bike?

Friction helps stop a bike when you use the brakes, but it can be harmful by slowing your speed down, if you hit the brakes hard at the sudden moment, you could possibly tumble and or fall. It also causes heat, so if not used correctly, that can be an injury. If you were to fall from the sudden stop, you could possibly hit the ground which can cause a knee injury and or scrapes.

Friction is harmful, because if you fall off a bike and friction occurs, you could get scraped/burned BADLY, or if you go on too high of a speed, your tires could burn off due to friction.
I hope I helped! =D