Why use a Microcontroller or a Microprocessor.
The truth is that we can do anything
with any of them. The right choice is to know about the final application and which of them will do better the job.
Microcontrollers are known to be compactest vs Microcprocessors and include in its chipset memories (RAM, FLASH, EEPROM) and peripheric as IO ports, serial ports (CAN, RS232, I2C, MSSP, SPI), Ethernet while the Microprocessor is designed to execute instruction fast and manage a lot of data. Microprocessor count with a data and address bus to manage memories and peripheric like Monitors, USB, Ethernet, KVM  that they are not included in its chipset. 

The Microcontrollers are recommended in applications with specific task that involving little processing, field measurements and often the designer uses a RTOS to develop the firmware. 
The Microprocessors usualy are used to PC applicationswhere makes necessary to process and save a lot data, manage, RAM memories, internal and external HD, Monitor, Keyboards, mouses, SD memories beetween others. Often the designer uses to develop the software an Operative System as Linux, Windows, etc.