Actually they don't. Today you can get belt driven bikes, or shaft driven bikes. And back in the days there used to be direct-drive bikes with the pedals solidly bolted to the wheel axle. There's also been a treadle-driven bicycle.
But direct-drive makes it awkward to find a comfortable riding position, and to get any speed you end up with really big drive wheels. Treadle-drive needs more moving parts interacting. Transmission shaft bikes tend to experience"wind-up" when being pedalled hard, and quite a bit of power is lost in the type of gearing that is needed in the power transfer train. Belt drive can be quite good, but as the belts can't be opened or cut to size, these bikes usually need special frame designs.
A chain is easier to work with than a belt, has simpler mechanics than a treadle, less mechanical losses than a shaft, and better speed/fit options than a direct drive, so it became the most common design.
To move the pedalling position from the hub of the wheel to a place where it'd work better. Also to provide a gear ratio allowing the rear wheel to turn faster than the pedals.
to reduce wear and unwanted friction.
Answer: LUbricate the Chain (Not using WD40) Keep Tyres Inflated Keep All Bearings adjusted and greased.
if you don't keep your chain and gears on your bike greased it will rust and come off.
a chain
Bicycle chain
no it is a lever
bicycle
It protects the chain from rain and road grime and it protects your trouser legs from chain gunk.
pulley
The chain is wrapped around a cogwheel, which could be described as a pulley.
A deraileur is the device that shifts the chain to different cogs on a bicycle.
The pedals, crank, chain, sprockets, and wheels. Along with the rider, they all work together to make the bicycle go.
It's a type of chain known as a roller chain that is used to transfer power from the cranks to the rear wheel of the bike.