Transistors can be used as a switch. A small base current is used to switch on a much larger collector current.

The diode allows the inductor current to die away slowly preventing a damaging back EMF that would otherwise destroy the switching transistor.

Inductive loads include

• motors
• stepper motors
• power distrubution networks - In this Google Video you can see a half megavolt circuit being turned off.
• relays
• buzzers (that include a coil)
• any other device that contains a coil and where sudden changes of current could occur.