Fuel & air mix together more easily and more thoroughly when the intake air is warm. The air is warmed in the intake manifold which gets its heat from the engine.** When the engine is running below the designed operating temperature, the fuel mixture must be richened, (choked on carbureted engines) so the engine will run acceptably. The thermosat begins to open at 195F, and provided the cooling system is doing its job correctly, coolant should regulate very close to 200F. The most common cause for coolant temperature being low, is a thermostat that does not close completely, or opens too early. So, if you're at 180F, engine performance, fuel mileage, and heater output are all adversely affected. Check the thermostat

* This is why "cold air intake systems", by themselves, are totally worthless.