The "XLR" connector is also known as a "3-pin" because it has, well, three pins. The third wire gives an XLR several advantages.

Advantages:

1) You can run an XLR cable for hundreds of feet without any significant loss of signal. Conversely, two-wire cables are limited to something under 50 feet.

2) XLR's are less likely to pick up outside interference than two-wire cables.

3) XLR cables have a little lock built in so they won't accidentally pull apart.

Because of these advantages, all high-end television production uses XLR cables for audio. The problem with XLR connectors is that they are big and bulky. Smaller gear needed a more compact solution--and thus the increasing use of the RCA and other connectors.