Hi Dan, Looks like L50 got ya all covered in his explanations of the relay use vs. not....
Electrical stuff has been my lifes work since I was about 16 starting work at Atari Inc. being a test tech on their computer games way back when... my job was to play games and 'test' them. But I grew up in the silicone valley when it was just starting out so I figured that electronics or something associated with it might be a good calling for me... turns out I was right almost 40 years later....
Relays are exactly how you described - a switch for a switch... but its a BIG switch for a small or tiny switch that uses a small or tiny wire. You can basically operate a 10, 20, 30, 40 50, 70 100 amp load off of a micro switch that only has an ampacity capability of less than .01 amp.... Therefore your dash panel doesnt look like a hydro electric switching facility to cover your intended load.
Also anytime you have a 'motor' load for a fan motor, a pump motor, a window motor, etc. that motor has a startup load that is much greater than its running amperage. So a relay takes on that potential EXTRA load without affecting other circuits drawing off the same fuse panel.
A fuse is a protection method to protect your circuit wiring from burning, the fuse burns first when it becomes overloaded. A relay is made to TAKE the extra load by the virtue of the size of the contacts inside of it. The contacts are rated for 20-40 amps, 50-70 amps, and bigger too. The wire normally used for the primary switching circuit can be as small as 18 GA (my minimum) but Ive seen 22-26 gauge also used without a problem.
EXAMPLE: Try running a piece of 26 gauge wire between a battery (+) and a starter motor and see how long it takes to burn it up....
Now run that same 26 GA wire thru a relay that has a 70 AMP capacity and the load side wire as a 6 gauge and it will work all day long.
The other reason to use relays is your switching wires can be as long as you like in the smaller gauge wire and the load side wires (bigger ones) can be shorter. This makes for less expensive electrical installations....
if you need more info just ring!