A microcontroller is a device which has an inbuilt processor surrounded by few dedicated hardware modules. Once the microcontroller initializes them they start operating on their own. In case of an ADC it will do the sampling and digital to analog conversion all by itself and keep the converted data in its buffer so that the microcontroller can read that later. The advantage of this kind of implementation is that the microcontroller is free to do other tasks during that time and hence increase the overall efficiency. That was the case of hardware modules or peripherals inside a microcontroller which increases the processing efficiency of the built in processor. The efficiency can increase even more if the external hardware attached to the microcontroller can also does lot more tasks by their own without depending the microcontroller.