Microwaves are useful to engineers (and hence in everyday life) for a variety of reasons. Chiefly, they are these:
- They are absorbed by water and some fat molecules, causing the release of thermal energy. But they penetrate most anything else (except metal). The idea that microwaves can cook or reheat food, reaching all parts simultaneously, was by no means the first one to put them to good use, but it is the one most people know about. Read all about how micros nuke food.
- Microwaves are reflected by metal. They are thus easy to direct and focus. This is also the original reason for research into them. Radar detects metal objects by the radiation they reflect. Additionally, this fact and the magnetron's tendency to do spectacular things when overloaded are the "science" behind many microwave experiments.
- Their wavelengths (millimeters to centimeters) are around the sizes of everyday objects. This means that reflectors, waveguides, etc. only need to be precisely manufactured on those scales. To direct microwaves, pretty standard metal tubing will do, as the precision required is only around the scale of the relevant wavelength; the reflector for a receiver or transmitter can be an easy-to-make metal horn. Much shorter wavelengths often require precise instrumentation, and much longer ones have completely different interactions with everyday (millimeter-to-meter-sized) objects.
- Microwaves are easily generated by a magnetron, pretty much at the frequency of choice. (Visible lightsources are often limited to one or several particular frequencies.)
- Background terrestrial-origin "noise" is minimal in (much of) the microwave spectrum. This makes it an ideal spectrum to search for astrophysical phenomena, or (if you're so inclined) to conduct the SETI.
- Microwaves carry a fair distance through lower layers of the atmosphere before dispersion, absorption, reflection and whatever distorts them. This makes them a good choice for communications.