Sometimes it's really useful to accurately control the speed of a motor. Usually it's quite difficult, requiring a powerful transistor to switch power for the motor, and a photochopper to measure the speed. However, the fans you find in newer PCs (the ones with 4 wires) have control electronics and a position sensor built in! You could easily cut off the fan blades and use the motors for whatever you want!
to control the fan speed in real-time!