A pedometer is useful in two ways:
In monitoring activity on a day to day basis you can compare your activity level against the 10,000 step recommendation. Simply by making you are aware of how active you are (and how active you should be), this information can help motivate you.
Armed with this information, you can set yourself a hierarchy of goals. If you are leading a sedentary lifestyle (i.e., someone who takes under 5,000 steps/day), then you can set a goal to increase your daily step-count gradually by adding an extra 2,000 steps into your routine. You should also set a longer-term goal to increase your step count to 10,000 steps.