The belt keeps the brush roller (agitator) spinning to help pick up dirt and debris. Most people think that you only need to change the belt (or band as some people call it) when it breaks and the machine stops picking up. Far from the truth. Most vacuum belts will stretch and/or wear so when the machine touches the carpet surface, it will either slow down considerably or completely stop spinning.

A good way to check that your belt is operating in your vacuum correctly is to put the upright (or power nozzle from a canister type) on the carpet as if you are getting ready to vacuum and with the power on, use your foot and apply a little pressure on top of the front part of the base. If you hear it slow down considerably or stop completely, you need a belt change. This is assuming everything else is working properly (machine has power, brush roller spins, etc.).