Chirpy,
That's what a full wave rectifier normally is used for but it has nothing to do with this application.

Here it is being used to provide a circulating path for the motor current when the contact on either side switches from one state to the other.
* Lets say the left contact is in the "up position" and the right contact in the "down position".
* The current is flowing from left to right thru the motor.
* Now the left hand side contact is switched to its down position in order to stop the motor.
* Just as the contact opens the inductance of the winding in the motor will cause the voltage on the "left side" to rise abrubtly, if the diodes wasn't there this voltage would casue an arc to develop across the now open left contact. Because of the diodes the voltage spike is clamped because there's a path thru the motor, down thru the right hand contact and back up thru the left hand side diode.

The reason for using a bridge rectifier like that is, I guess, due the spade terminals making it easy to connect when using an automotive type relay like the one in the picture. Two single diodes would work just as fine (and be cheaper).

/Henrik.