On gasoline engines the thermostatic air cleaner is designed to maintain a specific air temp. If the incoming air temp on an a gasoline engine is too cold especially on carb or throttle body injection the gasoline wont atomize proporly. Resulting in incomplete combustion and high hydrocarbon emissions.
Heat risers were designed to force hot exhaust gases through a passage in the intake manifold. This heated up the area under the carburator promoting fuel atomization at low engine temeratures.
Anyone that has owned a carburated V8 dodge in the northwest can probably talk all day about carb icing. Heated intakes can help with this but there is that first few minutes where you wonder if that stupid thing is going to stay running.