This allows you to adjust your gear ratio.
The more teeth on a spur gear, the quicker you will accelerate. Fewer teeth causes the opposite, you accelerate slower, but hit a higher top speed.
The more teeth on a pinion, the higher top speed you will have, the fewer teeth on a pinion, the quicker you will accelerate.
This would be a simple explanation for single speed transmission's, but most nitro touring cars have two speeds and so things get more difficult. Beside the change in performance, switching to new gears can also cause other problems.
My MTX2 is running a 16/20 pinion and a 48(?)/ 44(?) spur gear combo. If I wanted more acceleration of the line, I would want to get a larger first gear spur or smaller first gear pinion. However, since a spur gear with more teeth will have a larger circumference than a spur with fewer teeth, I will need to also adjust one of the other three gears in order for them to all mate properly.
EX: 44/48 won't work with 17/20, but
a 45/48 will work with a 17/20.
Hope this helps, (and I typed everything correctly) please tell me if I wrote something wrong.