A bump rubber is used to keep the car from bottoming out and to add spring rate and therefore dynamic corner weight when the chassis travels far enough to get into the bump rubber. Generally bump rubbers are used on the left rear shock.
It is used on winged racing in some situations to keep the car from bottoming out on the left rear on entry which will kill traction. However, because it also adds a progressive spring rate on the left rear can also loosen the car because it unloads the right rear on entry. See the Rethink Dirt paper in the Tech Department of the website for a more clear understanding.
A bump rubber can also be used on the left rear of a wingless car to add traction on corner exit. As the car squats on the left rear under acceleration, if set correctly, it will compress the bump rubber and add a lot of left rear weight, which can, if not excessive, add traction.The timing of when the chassis hits the bump rubber is very critical. If it hits too early, the increase in left rear spring rate will make the car very erratic. If it doesn't get into it soon enough, the car will still bottom out, or not increase spring rate, and the bump rubber will not serve its purpose.
By adding spacers (generally 1/8") you can adjust when the car hits the bump rubber. Shackle length will also affect the bump rubber hit height.