The Faraday cage shows up whenever you are trying to isolate an area from electromagnetic flux (fields, EM radiation, what-have-you). The two most common reasons for doing this on a macro scale are:

  1. EMP Protection: In order to shield electronic equipment from extreme transient electromagnetic fields (usually produced by atomic detonation but not necessarily) you can build a Faraday cage into the outer case of the equipment in question.
  2. Defeating radio or other EM transmissions: Most graphically illustrated in the movie Enemy of the State in 'The Jar,' a Faraday cage can serve to prevent any transmissions from entering or exiting the enclosed area. This makes bugging or otherwise electronically snooping on that area extremely difficult.