The brakes! And the tires (no friction = no traction).
Brakes are meant to have a lot of friction and it's useful.
The bearings
By creating friction between a moving part and a stationary part.
Any bicycle part that makes contact with another part but needs to move to function must have friction reduced. The wheels bearings, steering head, pedal bearings in both the hub and pedal, and the chain links all are designed for minimum friction but they also need lubricant.
Friction isn't useful for doing things like sliding a heavy couch to another part of a room.
In the bearings that go between the stationary part - the axle, and the rotating part - the hub.
In any part that you want to keep moving, like a spinning wheel, friction is just a nuisance.
Same as on a car. Applying the brakes means that something stationary is pressed against a moving part. This generates friction, which turns speed into heat.It produces friction, because when the brake touches the rim of the bicycle, the bicycle slows down. (thats called friction) And after a while the bicycle stopsA stationary part is brought into contact with a rotating part. The resulting friction slows the bike down.
A bicycle has high friction at the brakes, first and foremost. Then at places where the rider holds on to the bike, Grips and pedals. Between tires and road.
In the bearings, where you want as little friction as possible.
At the contact between the rubber of the wheels and the road surface. At the contact between the wheel rim and the brake pads.
Friction is very useful but it has its own disadvantages. Friction causes wear and tear. Friction spoils the soles of our shoes. friction reduces speed This is all wrong some not useful types of friction are are when airplanes fly the friction causes heat that materials that will withstand which are costly.