Did you know that your microwave might have a microphone in it? If you purchased a quality microwave, chances are it comes with a sensor that pics up noises from inside its chamber. This sensor works in conjunction to a moisture meter to determine the amount of steam within, allowing the microwave to figure out appropriate times and strengths to microwave something at. This is why the "popcorn" button exists, as a microwave with these sensors will listen to the popcorn and gauge how much steam has escaped from it to figure out an optimal microwave time. This is the same for the "Defrost" and "Reheat" buttons, allowing you to defrost frozen proteins or reheat leftovers without worry of cooking them or drying them out.
Not all microwaves are like that though. Cheaper ones, while still having the popcorn/defrost/reheat buttons or options, instead just use a preprogrammed timer instead of any sensors. If you want to use these features, be sure to know what model you have or even give them a test yourself next chance you get. This is why, despite having a popcorn button, most popcorn brands yell at you never to use it. Not because there's a conspiracy between popcorn makers and microwave makers, but because they don't know what model of microwave you have and thus err on the side of caution by instructing you with the tried-and-true method of just listening to the kernels pop.
