Useful - friction gives us a hold on things. This allows us to hold objects, walk, drive... Accidents can be prevented because of friction. This stops or slows down the motion of the car.
Harmful - a boy running on the road suddenly fell. He got a scrape on his knee because the friction between his knee and the road broke his skin. Other than that, friction is a force which can cause the requirement of force to move a car to increase. This is because the force needed to cause a forward motion in a car must be more than the frictional force between the tyres and the road.
I hope it was useful.
