It's a supplementary source of instant power if the voltage drops for a fraction of a second when you give it the beans and the MOSFETS engage full on. Some ESCs are sensitive and some batteries drop monetarily for whatever reason. You need a fast capacitor too, that's why they're not as cheap as equivalent garden variety capacitors you can pick up at Tandy's and such. You don't need to buy at your local shop, there are other sources and they may be cheaper, but you need to make sure you compare apples with apples.
Team Greasy Weasel
The best upgrade to any car is some driver skill.