you pea andpoop all the time its like DON'T STOP TILL U GET ENOUGH lol the real answer is they use it for if they are trying to find what some thing means or they can use it to get exercise...
He received the invisibility cloak which was anonymously sent to him by the headmaster of Hogwarts, professor Dumbledore. This was useful because it allowed him to go about the corridors at night...
Friction is important for all sports.
Without it it becomes really hard to accelerate or deaccelerate.
For example a 100m sprinter.
It would be great for him if there wasnt any air resistance...