I guess you meant to use heater to do it.

It may work in winter when the fog is caused by cold windshield and outside humidity is generally low. However when it rains the humidity is too high for the heater to work. In summer it is a pain to do this and it takes forever to clear up the windshield.

A/C can remove the moist from the air, which the heater can't do.

I used to have a car without A/C and it was a pain to drive in rain in the southern USA.