It's also because it works faster and the entire energy is focussed directly on food rather than heating containers or surrounding air. Microwave ovens don't use “radiation”, instead they use radio waves. It's just that the source of microwaves is a very bright source, and hence it's able to cook food.
Also question is, what are the uses of microwave oven?
Microwaves with certain wavelengths are absorbed by water molecules and can be used for cooking. Water in the food absorbs the microwave radiation, which causes the water to heat up and cook the food. The water in living cells can also absorb microwave radiation.
How does a microwave work?
Microwave ovens are so quick and efficient because they channel heat energy directly to the molecules (tiny particles) inside food. Microwaves heat food like the sun heats your face—by radiation. A microwave is much like the electromagnetic waves that zap through the air from TV and radio transmitters.