Question

where heat produce by friction is useful.

Answer 1

Rubbing you're hands when you're cold.  The matchstick when rubbed against matchbox.

Answer 2

Friction converts useful kinetic energy into thermal energy or heat. Friction gets a bad rap for turning useful energy into useless energy. it's true that a lot of kinetic energy in the world is lost into the Atmosphere as Heat. but heat isn't always useless.