Vacuum belts connect to the brush roller and assist in collecting debris. On the other side of the belt is usually the drive shaft or motor pulley. After regular usage, the belt will stretch out, lessening the tension on the brush roller. Lesser tension diminishes the brush roller's turn rate and speed. This means more debris left in your carpet and air.
For optimal performance, the belt should be changed out every 3 months. Although the belt might look in great condition, it might be putting a strain on your vacuum if it is loosing grip and causing the pulley stress.
To find the belt part number compatible with your vacuum, please refer to your manufacturer manual.