Most ESCs today have what is termed BEC or Battery Eliminator Circuit inbuilt. A few years ago, RC aircraft would have two batteries onboard - one for the receiver and another for the motors. The BEC eliminated the need for a separate battery for the receiver and reduced the need for RC aircraft to carry the additional battery.
The BEC also steps down the voltage from the lipo batteries before the supply is fed into the receiver to around 6 volts - which is what the typical receiver is designed to handle.
In addition to reducing weight by eliminating the need for a separate battery for the receiver, using a BEC has another significant advantage - you do not have to remember to charge the receiver battery. You only have one battery mounted on the quad - and if that is charged then the quad will fly. The receiver will function, the servos will work and the motors will run. You do not have to worry about the receiver battery as the BEC eliminates the need for one.
While the advantages of using a ESC with an inbuilt BEC are obvious, you would want to be aware of the risks as well...
You see, if you are using a ESC with an inbuilt BEC, you will be depending a lot on the reliability of your ESC. Your ESC would then be powering your motors, the receiver and any servos you might be using.
And if the ESC were to fail mid flight for whatever reason, your drone will crash. There will be no way you will be able to prevent that.
One of the most common reasons why ESCs fail is because of overloading. If you have too many devices connected to the ESC - and in case they draw more current than the ESC can handle, the ESC will fail.
So if you are planning to build an expensive drone with high resolution cameras and other electronics, you would want to be sure to minimize the risks associated with the ESCs failing - and ensure the ESCs never ever get overloaded in any condition. And the best way to do that would be to use an external BEC (also called UBEC or Ultimate BEC). An external BEC will handle some of the load so that the ESC doesn't get overloaded.
By using an external BEC to power the receiver, even if the ESC fails, the receiver will still work. Sometimes, that may be a good thing. Even if one of the motors fails, some flight controllers have algorithms that can detect the problem, identify which motor failed and then take corrective action to prevent a crash. These kinds of algorithms however are not available in most low cost flight controllers. Most hexacopters shut down the motor opposite the one that fails and the craft flies like a quadcopter.
Usually, ESCs with inbuilt BECs are used only in very small quadcopters where weight needs to be kept at the absolute minimum. Larger models usually have a separate BEC and do not use ESCs with built in BECs.