I've always liked this analogy!
Think about taking a trip on an airplane. Before taking off, the pilot has a very clear destination in mind, which hopefully coincides with yours, and a flight plan to get there. The plane takes off at the appointed hour toward that predetermined destination. But in fact, the plane is off course at least 90 percent of the time. Weather conditions, turbulence, and other factors cause it to get off track. However, feedback is given to the pilot constantly, who then makes course corrections and keeps coming back to the exact flight plan, bringing the plane back on course. And often, the plane arrives at the destination on time. It’s amazing. Think of it. Leaving on time, arriving on time, but off course 90 percent of the time. If you can create this image of an airplane, a destination, and a flight plan in your mind, then you understand the purpose of a personal mission statement. It is the picture of where you want to end up—that is, your destination is the values you want to live your life by. Even if you are off course much or most of the time but still hang on to your sense of hope and your vision, you will eventually arrive at your destination. You will arrive at your destination and usually on time. That’s the whole point—we just get back on course.
― Stephen R. Covey