Overview
Balanced or XLR connections are the preferred way to make an audio connection in a recording studio.  In audiophile circles, there is debate over the audio advantages of balanced vs. unbalanced (RCA) connections.  XLR connections are preferred in the studio environment because of their ability to make a quiet, reliable connection, even over long cables.  XLR connectors are almost always found on pro audio equipment, specifically microphones.

Balanced Home Theater Systems
Most home theater systems use unbalanced (RCA) audio connections.  The main reason for this is physical real estate on the back of an AV receiver or AV preamp.  In today's systems, there isn't that much room for all of the inputs and 7.1 audio outputs, even on receivers.