The typical microcontroller used for a digital controlled power supply will have a good deal of extra computation time between running the control loops for a power supply. Some microcontrollers actually come with computational acceleration hardware to speed up the power supply computation process. While this could mean that a little slower speed, smaller, less functionality microprocessors can be used to manage and control only the power supply. It also means that more of the system processes can be managed and controlled by the microprocessor with the same central processing unit and overall configuration with extra modules, memory, and functionality.
Microcontroller families generally are the same base model chip with more or less bonuses to best fit the customers cost vs. functionality needs. Microcontrollers can come with different amounts RAM, EEPROM, and Flash memory and different numbers of pulse width modulators and modules, analog-to-digital converter (ADC) channels and converter cores, digital input/output (IO) pins, and communication ports and other components. Many times a pin is connected to one or more of the functional blocks (PWM, ADC, etc.) and can be configured to take advantage of one of these connections.
Depending on the size and needs of a system, a microprocessor can be chosen that will run the entire system as well as the power supply or only the power supply. Controlling the power supply, switching loads on and off, acting as a motor controller, communicating with personal computers and other devices, and monitoring voltages, currents, temperature, etc. can all be done with a single microcontroller.
Systems that require many different voltages or multiple power supplies can have a single microcontroller chosen to control and manage all the supplies together as its own subsystem. In addition to reducing size and cost, a microcontroller can allow the power supply subsystem to communicate and work easily with the rest of the system. This would allow a designer to run the system and interact with the power supply subsystem without worrying about the control and management of the power supplies at the same time. Even using individual microcontrollers to control each power supply in the system would allow for a level of communication that would be very hard to achieve otherwise.