As you drive your vehicle on city streets and in traffic, your engine tends to heat up a little more. Thus, it needs more air flow to keep everything cool. As you go onto the highway, the additional air movement over the radiator at speed requires less intervention from the radiator fan. This is exactly what the fan clutch does. It adjusts the speed of the fan based on the rotational speed of the engine. When unneeded, the fan clutch reduces rotational load and drag. If you have a mechanical fan and your vehicle runs a little hotter than it should in traffic, this may be your solution.