Microcontrollers evolved from specialized processors that had only a CPU on a chip. The earliest design concept was to make computer systems smaller by integrating a large number of peripherals and CPUs into a single chip, making it easier to integrate them into complex and size-critical control devices.
The microcontroller is for control, writing a program to the microcontroller chip to let the microcontroller automatically control something. It's really a simplified computer system.
It is a small embedded chip that can be programmed to achieve some of the functions that the user wants to achieve.