The heating elements are sized with enough BTU's to heat the object; window, pitot tube, prop blades, etc, with the aircraft moving through the air at speed. When the aircraft is sitting on the ground and not moving very fast they tend to overheat, in some cases causing damage to the object being heated. Some have overheat protection but if that fails you cook your goodies.
I turn on the pitot heat before entering the runway as a part of my final scan using SPITT - Switches (lights), Pitot Heat, Instruments (DG to comp[Mod - Happy Thoughts] check, OBS to first intercept, etc.), Transpond