Faceplates can be configured differently in different locations. For example, you may want one UTP and three coax connectors in a media room faceplate. We see a scenario calling for just this configuration. Extra jacks also allow for moving applications from one room to another, and for growth in applications. Not all jacks are active at any one time.
Jacks are cross-connected to applications as required.
Some jacks may never be used. They are just a hedge against an uncertain future. Different applications use different pins. Plain old telephone service uses pins 4 and 5. A second phone line to the same jack would use pins 3 and 6.