A free field microphone is used, as its name suggests, in a free field acousitic environment.  This means that you should have ideally just one source of sound and there should be nothing that causes reflections of the sound waves from the source.  The ideal free field environment is outside in an open area.  An anechoic chamber will provide free field conditions down to the cut off frequency of the chamber.

A random incidence microphone is the sensor of choice where you have multiple sound sources or relfective surfaces.  An example would be acoustic measurements in a machine shop.