An electrical junction box is a very important part of your electrical wiring system. Without a junction box, you simply cannot have that many different types of electrical outlets, switches, or data hookups in a room.
The electrical junction box is usually a distribution center of a sort, where a common feed coming from the breaker is hooked up to multiple cables inside a properly sized enclosure box so that the power can be re-routed to each their respective circuits. Sometimes, however, it can be added into a line that's not long enough, to permit the splicing of the wires inside a safe enclosure. These junction boxes are commonly used in basements, inside drop ceilings, or around the house usually hidden behind the drywall, or paneling as they serve no esthetic function.
Metal or Plastic
An electrical junction box can be made of either metal or plastic. They usually measure 2½ inches deep or 3½ inches deep. The larger of the two is able to hold up to five different wires, while the smaller only holds three wires.
Needed for Several Installations
If you are going to be installing several different types of electrical components in a room, then you will need to tie into the existing wiring somewhere. A junction box makes that easier by allowing you to connect directly to the main breaker. This negates the need to running wire through the entire house back to the breaker, providing that the power requirements of those components don't exceed the circuit breaker's power rating.