At work we sometimes use them with an Arduino; the Pi tells the Arduino what to do ( for instance turn a stepper motor ) and the Arduino sends the pulses out to the stepper motor.
Another use is to connect it up to sensors and the Ethernet and the Pi sends the information from a sensor to another computer in a different location. A lot cheaper than a full size computer.
A guy at work has one connected up to a microscope and uses a Pi camera to send the information to a monitor.