The idea behind relays is to keep the power flow closer to the accessories, and only run the larger wiring as short as possible.
For instance, one of the earlier posters notes that he uses CAT5 twisted pair cable for his overhead switch console connection. This cable is NOT suitable for high power applications.
Put another way, a relay is a remotely controlled switch. Yes, this means that for a lot of this stuff, there are 2 switches in the circuitry. The relay on the sense side has the overhead switch in series. The power side has 12ga or 10ga wiring to hand the big power requirements.
Here's another simple example: relay controlled lighting harnesses. The original headlight cabling is connected into the sense side of the relay. This wiring is known for being somewhat undersized and long as it runs all the way into the cab into the headlight switch. As a result, the headlights often don't see full battery voltage due to the wiring drop. Once a relay harness is used, the wiring from headlight to battery is now much shorter, and much heavier, so there is practically no voltage drop.
Same concept stolen from fitch. 
