The reason you put a glass of water next to your pizza in the microwave is so that the pizza does not get soggy, overly chewy and creates a more even heating process for the pizza.
The water in the glass acts as a source of moisture for the pizza and helps to distribute the energy of the microwave evenly through the microwave cavity.
The water in the microwave makes for a slightly more humid environment and helps prevent your pizza crust from drying out and becoming overly hard or chewy and also helps to keep the pizza toppings from becoming tough or overly dry.
The reason why you shouldn't reheat pizza in the microwave is because the microwave can make the pizza crust soggy and the cheese can turn rubbery.
Microwaving pizza to reheat it or cook it can compromise the overall flavor and the texture of the pizza.
The microwaves heating method relies on water molecules that can lead to uneven heating and moisture distribution of the pizza and results in a poor tasting and less desirable pizza.
The heat of the microwave can cause the pizza crust to become soggy and soft and lose it's crispness and texture.
Microwaving pizza can also cause uneven heating of the pizza where the pizza toppings often become hot while the crust remains cold.
If the pizza has cheese it can become rubbery and lose it's melty and stringy texture when it is reheated in the microwave.
The flavor of the pizza can be diminished when it's microwaved due to uneven heating and changes in texture.
Heating the pizza in a toaster oven, oven or even an air fryer would be much better.
However I have sometimes reheated a slice of pizza in the microwave for 1 minute and it turned out okay.
Just don't heat it too long or it could also burn and become hard.
I prefer to heat the pizza in my toaster oven or air fryer but if I just want to heat it quickly I use the microwave.