What are microwave filters used for?

Microwave filters are used to achieve the desiered bandwith or
ferequency of the microwave (high frequency radio waves).  These
filters may be used to eliminate undesiered high frequency radiation
or to controll the wave for various applications.  I suggest looking
them up in "The Way Things Work" which you may be able to find at your
local library or even just pick up at a nearby book store.  I'll see
what my copy says on micro wave filters.  It will give a list of
applications and technical drawings with the detailed descriptions of
the devices function.

A typical application is to select different bands in satellite
transponders. If you are not that technical... in the satellites
bouncing the TV signals emited from earth, there they use them. In
Cellphone antennas, to select the range of frquency of interest and
thus remove noise out of that band... any kind of receptor in the
microwaves frequencies (GHzs) will have one microwave filter.