I have an 2003 SO and was wondering what the cable hooked to the fan hub is for? What is it's purpose?

 

It allows the computer to engage and disengage the fan clutch. Fan engagement is based on engine temp, under hood temp, road speed/engine rpm (for increased fuel economy), exhaust brake operation (fan kicks in to help slow the truck) and A/C pressure (makes the A/C work better) on "big truck" applications. I don't know how many parameters the Dodge PCM looks at when controlling the fan.