Pinions alter the gear ratio. The gear ratio needs altering so that the available torque from the motor is used effectively. It would be unusual to change ratio for track surfaces, but it would be usual to try different ratios for different track layouts.
Ratio changes usually go with motor changes. The less winds the motor has (6.5 turn is less than 10.5 turn, for example) the smaller the pinion must be fitted. Also, ratio changes go with the amount of fixed timing you have on the motor; more timing usually needs a smaller pinion.
If you are not changing your motor you are usually not changing you gear ratio and therefore not changing your pinion. If you don't have a range of motors to use don't bother buying a load of pinions. When you do change your motor ask around for what ratio people are running, that allows you to choose the right pinion and saves buying lots of them. HTH
