Heated trucks are the same as reefer trucks, only they have their temperature controls set to a warmer degree. Their temperature settings allow them to act as both heaters and coolers of items. The heating side of the temperature dial is especially good during winter months and in areas where temperatures can drop well below freezing, such as mountainous areas that are at a higher and cooler level of altitude.
Warmer than freezing temperatures are good for all those products that need to be kept cool but not frozen. Things like milk, wine and cheese would be good examples of things that would require a more delicate temperature control than something like frozen meals or ice cream.
Produce also often requires more specialized temperature controls than other cargo. It needs to be kept fresh without being frozen. Fortunately, with a heated truck, you can simply set the temperature to whatever you need it at and it will keep, no matter what the outside temp is.