Hot spots. The microwaves will sort of clump together in places within the rectangular box. These clumps will be super heating a tiny bit of the volume. Spinning the food spreads out the super heating around the lump of food. Without spinning it can destroy stuff.

Recently while heating a tub of chocolate buttons to make a topping, the tub caught on the side and stopped spinning. The side of the tub super heated and melted letting liquid chocolate, still being heated, run all through the bottom of the microwave oven. It was an awful thing to clean out.