[MagmaJct]

I think the reason intake is pre-heated has to do with the laws of thermodynamics.

In order for a liquid to vaporize, heat must be absorbed in the process. This, incidentally is how air conditioning works in the evaporator coil. When fuel is atomized in the carburetor, then exposed to the lower than atmospheric pressure of the intake manifold, it absorbs heat and vaporizes.

If there isn't enough heat to be absorbed, a certain percentage of the atomized fuel fails to vaporizes, and collects in the intake manifold. In pre-emissions era, carburetors were tuned rich, which compensated for this loss of fuel that collected on these surfaces. This fuel, eventually made it into the combustion chambers, but in near liquid form, it burned poorly and contributed to HC pollution.

The solution was/is to pre-heat the incoming air. By providing more heat to the atomized fuel, a greater percentage of it would vaporize, and contribute to the power of combustion. Because more fuel was vaporized, the carburetors could then be tuned leaner.

For this reason, many incorrectly believe removing the pre-heat will result in a richer mixture and more power. Not necessarily so! (Although, cool intake does provide a more dense charge of air, which can increase power, but that's beyond the scope of this write up!)

Port fuel injection did away with pre-heat, as fuel injectors atomize the fuel right above the intake valves. There is plenty of heat, and very little surface for the fuel to collect/condense on.

Regards,
Mike

Edit: For typos!