What is the main purpose of SPDT and DPDT switches?

The number of poles refers to how may separate signals or if you will, wires you switch when the switch is activated or deactivated. For example your typical light switch just switches the hot wire and turns the light on or off and is called a SPST switch.

The SPDT switches a signal to either of two circuits. So for example, if you had hot connected to the input and a light connected to each output (Throw), you would have one light on and one off and each time you throw the switch the lights switch which is off and which is on.

The DPDT can switch two separate signals at the same time and each goes to one of two separate circuits. If you had two separate breakers and four light bulbs you could hook up one breaker hot to one pole and the other breaker to the second pole. If you had four light bulbs each connected to each of two switch outputs, then one bulb from each breaker would be on and when switched the other bulb on each breaker would be on.

For diagrams just search for SPDT and DPDT.