[ischmitz]

An inductive load will produce a spike when a circuit is interrupted. That is how a Kettering ignition works. The coil induces a voltage spike to counteract the breakdown of the field.

This is why relays (or inductive loads in general) that are driven off a semiconductor have a diode across the coil to supress this spike. Modern automotive relays have these diodes built in and there is no need to add external diodes.