I had seen a machine (electro-surgical generator) used for surgery purposes; it had a digital interface having 7 segment displays on the front board. It was using a PIC18f452 and a Atmel EEPROM 24C64. What's the use of a microcontroller in such machine? Traditional (electro-surgical generator) machines use knobs and switches; why is the micro-controller actually used?
There's never a case where you have to have a microcontroller, at least not technically. Anything a microcontroller does can also be done by an appropriate combination of discrete components. What the microcontroller buys you is size and flexibility. The microcontroller does the job in a smaller package than the discrete components (which sometimes makes the difference between practical and not), and can be programmed to do a different job vastly more easily than a discrete component circuit can be redesigned.
There's also the fact that many engineers, especially younger ones, know microcontrollers far better than we know discrete components. I wouldn't know what to do with a 555 timer or a hex decoder or half the parts on the more dusty shelves around here. But I know I can accomplish all those tasks in a smaller chip. So if you asked me to design something like that, it would have a microcontroller just because that's what I know.