Cigarette lighter heaters are designed to convert energy from your car's electrical system into heated air. The problem is they can only draw so much power, so the resulting heat is paltry. That also means they cannot stand in for a standard vehicle heating system.
Car cigarette lighters are 12-volt outlets and most cigarette lighter heaters can deliver only around 150 watts of energy, which isn't much when compared to a common blow dryer, which usually blasts up to 2000 watts of heat.
In short, cigarette lighter heaters can produce heat, but not much and never in lieu of a traditional car heater. That makes them better suited for very simple things like defrosting the windshield or warming up hands.