Why is it necessary for the plate to move around in a microwave on a turntable, as opposed to staying still?

well, From this particular excerpt, I gather that older microwaves used to lack turntables,

Take steps to ensure that the microwave oven heats food evenly and does not leave underheated areas (cold spots) where bacteria might multiply and cause food poisoning. You can promote even heating in the microwave oven by:

* cutting food into small pieces for uniform cooking
* arranging items in a uniform manner
* adding a liquid (such as water, juice or gravy) to solid foods
* stopping part way through cooking to stir foods or rotate trays or containers
* covering food with a microwave-safe lid or with microwave-safe plastic wrap to trap steam
* following directions for "standing times". This helps ensure that heat is distributed uniformly, even after cooking.

Therefore, I assume that its primary purpose is to provide even heating.

http://cooking.stackexchange.com/questions/13357/why-does-a-microwave-rotate-the-food-being-cooked-and-is-there-ever-a-time-when

Confirmed, the microwaves leave gaps so the food must be turned so it reaches it all (satguru).

It seems that the primary purpose of the glass turntable is to prevent uneven heating of the food.

Uneven heating in microwaved food is partly due to the uneven distribution of microwave energy inside the oven, and partly due to the different rates of energy absorption in different parts of the food. The first problem is reduced by a stirrer, a type of fan that reflects microwave energy to different parts of the oven as it rotates, or by a turntable or carousel that turns the food; turntables, however, may still leave spots, such as the center of the oven, which receive uneven energy distribution.

https://en.wikipedia.org/wiki/Microwave_oven

If you are interested in knowing why a turntable rotates randomly (and a wealth of other MICROWAVE OVEN MYTHS) check out this really cool website:

http://amasci.com/weird/microwave/voltage3.html

Actually, here is a more scientific response to your question:

As the microwaves bounce around the inside of the cooking chamber, they tend to interfere with one another. There are usually regions in which the waves that follow various paths almost cancel one another and regions in which the waves reinforce one another. These regions don't cook food equally well. If the microwaves are canceled in one region, cooking will be slow there.

If the microwaves reinforce one another in another region, cooking will be fast there. If you simply leave food in one place and try to cook it in the microwaves, the cooking will be uneven. However, if the food is rotated continuously, these good and bad cooking regions will be blurred away so that the food will all cook at about the same speed.