The heaters are there to prevent internal condensation when the motors are used outdoors and/or in high humidity applications. The heaters turn on when the motors not running. Not all motors are used that way, so the heaters are an option, LV or MV. The cost of MV motors is so high that the added cost of heaters is relatively insignificant, so they are often ordered that way regardless. That's not 100% however.
In addition there are other ways of heating LV motors via low voltage transformers, solid state "motor winding heaters" and some VFDs. Those options are not available for MV motors for the most part.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington