J-Track , our java spacecraft tracker, uses state vectors to calculate spacecraft positions. Much of the replanning of the mission events uses a state vector as a starting point. State vectors also allow the ground to predict things like when and where the shuttle can land. Ground controllers use the state vector to predict which orbit, or rev, the shuttle will be over the appropriate landing site. An orbit adjustment, or burn, is a firing of the OMS engines to raise, lower or adjust timing of events, such as landing opportunities.
State vectors are as important to the ground support personnel as they are to the shuttle itself.
The shuttle needs to know where it is too. During any shuttle flight, updated state vectors are uplinked to the shuttle on a regular basis. The form of a state vector is just a way of identifying which set of orbit parameters is being used to define the orbit. All of these forms have one parameter in common, time. It is necesary to relate all the parameters of the state vector to a particular time point.