I thought these covers were more for the purpose of keeping your sauces from splattering in the microwave, less work to keep the microwave clean. I know from experience that the spaghetti and meatballs from a can likes to make a mess of my microwave. I usually cover my bowl with a paper towel.
Usually food doesn't take too long to microwave so it shouldn't lose too much heat in the process. Microwave ovens penetrate the food you are cooking, in a sense cooking them internally. This is a little different to covering a pot of boiling water on low heat to keep the heat in and to prevent raising the humidity of your house because usually we don't boil things in the microwave.
It might help if you are trying to steam vegetables in the microwave because it will keep some of that moisture in a smaller space but I don't usually steam my vegetables in the microwave so I don't know. I'd probably do that by covering them with a moist paper towel or something.
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