What is the purpose of unplugging the heater hose in a car?

You would actually install a valve to cut off the flow through the heater hose. If you just "unplugged" the hose, you'd spit out antifreeze everywhere. The purpose is to cut off hot engine coolant to the heater core. In the Summer months, you might get a couple degrees cooler air out of your AC system doing this. If you have a vehicle which uses a heater control valve, and that goes faulty, this is often what you'd have to do in order to get the AC to work. Barring that, you could pinch the heater hoses off with a set of hose clamps (preferable) or needle nose vice grips, tie them up real good with wire ties to ensure they don't come off and drop into the engine compartment, and run that way in lieu of installing the valve.