Why is a chain wheel important on a bike?

The power source on a bicycle is the rotation of the
pedals as they are pushed by the feet of the rider. As the pedals rotate, the chain
wheel also rotates at the same speed as the pedals since it is attached to the same axle
as the pedals. The chain on the bicycle is moved because the links of the chain are
meshed with the "teeth" on the chain wheel.

As the chain is
rotated by the chain wheel, that movement causes the chain wheel attached to the rear
tire axle to also rotate. The chain passes over one or more derailleur gears as it moves
around its path between the two chain wheels. These gears increase or decrease the speed
at which the rear chain wheel causes the rear axle (and therefore the attached rear
tire) to rotate. This is how the bicycle rider can adjust the speed and pressure that
must be exerted to make the bike move. If the chain wheel attached to the pedals didn't
move, the bike would move only in response to gravity as it rolled down a
hill.