Battery Eliminator Circuit
Most ESC have a BEC integrated into their circuitry. If you have a separate BEC is most likely because the ESC you have is not capable of handling the batteries you want to plug into the ESC to power the radio components.

For example, some people are hooking up 4S batteries into a stock Trex 450 ESC. The BEC in the ESC is not capable of handling the 4S voltage and feed it to the receiver. So some people bypass the BEC, still use the ESC to power the motor but power their radio components from the separate BEC.

I hope I was of help. Sometimes this gets confusing.

Rafael