Suppose you want to know how fast you traveled from the park to the library. As you moved, your speed varied from second to second. Therefore, in order to describe your speed, you must describe the average speed of the entire trip. Average speed is one reason. It is the distance an object moves divided by the time it takes to travel that distance. If you covered the 1 km distance to the library in 15 min, or 0.25 h, your average speed was 1 km / 0.25 h or 4 km / h.