Why does a microwave rotate the food being cooked? And is there ever a time when it shouldn't rotate?
Microwave ovens have a turntable because the microwaves themselves form what are known as 'standing waves'. This means there are essentially static columns of microwave energy inside the cabinet itself.

You can see this for yourself - spread a tray or plate with grated cheese, take the turntable out (or press that button), and zap it for a couple of minutes. You should see bands of melted cheese interleaved with unmelted cheese. Thus, the food is rotated to ensure even heating.