Richard is correct, assuming you have the A/C kit or at least part of it installed.
That fan is for when the A/C is running or when a thermostatic switch has it turn on to aid in cooling air flow. It's part of the A/C kit. With the A/C condenser heat exchanger mounted in front of the engine's radiator, flow is restricted compared to a non-A/C truck. When the engine is running really hot, this fan assists the engine-driven, viscous-coupling fan to ensure adequate air flow.
After I reinstalled my 97's A/C kit, in order to test the switch's proper operation, I just parked the truck and ran it for a while to make it hot and waited for the fan to come on. It was summer in FL and took a while. I remember it being about 215F on my VDO gauge, but it was a long time ago.
If your engine-driven fan seen in the engine bay isn't turning at all, then you have a problem.