[There’s a what-if xkcd](https://what-if.xkcd.com/131/) that covers this pretty well, but basically because the microwave is sending, well, waves of radiation to agitate water molecules in the air, food, and everything around it to create heat; if you visualize a sine wave and look at the mid point of a waveform between a peak and a valley, where the “0” line on the graph would be: where the microwaves pass through there is going to be a cold spot where things don’t heat as effectively as when the things go through a hot spot. The rotating platter is a way to get this to change.
This is also why, for instance, you can microwave a hot pocket and take one bite that is still frozen and then the next will be lava, and *also* also why it’s recommended, generally, to cook things on a lower setting for a longer time, and to let microwaved things sit a minute or two before serving/eating: giving things time to sit isn’t necessarily to cool them off, but to allow the system to reach equilibrium in some heated state, where the heat from the hot spots spreads out into the cold spots.
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