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. Ethernet uses pins 1,2,3 and 6. There are other applications that use pins 1,2,7 and 8. Commercial cabling standards call for the 8-position jack.