The skin of the metal container can become an antenna; Grounding primarily helps to keep the cage from becoming charged and perhaps re-radiating... More later hopefully.
Looks like the Houston *stros MVP 2017-2019?
This would lead one to think that maybe if you use the microwave as a faraday shield, you SHOULD leave it plugged in. Why? Because that third prong on the plug, the bottom one that's slightly longer and kind of rounded, serves as the chassis ground and 'should' (if your house wiring is relatively modern) provide a solid path to ground, which would include, at a minimum, an 8 foot copper or copper-clad rod stuck in the ground somewhere near where the power lines enter the house.
I think there out to be some other method of preventing ones' absent-minded self from turning on the microwave, but maintaining that chassis ground has to be a good thing.