when you boil liquids in your microwave. It may not be necessary, but it won’t hurt anything either.
Things are different in a microwave oven. The water gets hot but the container usually does not. There are no “boiling-bubbles” triggered by a hot metal pot. Without those bubbles to cool it, the temperature of the water rises far higher than 100C°. We call this “superheated water.” Superheated water is just waiting for some sort of trigger which will let bubbles form and allow boiling to commence. If the water becomes hot enough, a few bubbles will appear, but these quickly rise and burst, and the water isn’t cooled much at all. In the microwave oven, even if your mug of water is bubbling slightly, don’t trust it, since it’s temperature has risen so high above 100C° that bubbles are appearing spontaneously. If some unwitting victim should pour a soluble powder into the superheated water, this will carry thousands of tiny air bubbles into the water. Each of these micro bubbles expands into a 1cm steam bubble, and the result is a huge “explosion” of hot froth. It’s just like dumping ice cream into root beer, but the froth can be so violent that the hot water sprays into the air.