Here's why:

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* Average velocity is calculated by dividing the total displacement (change in position) by the total time taken.

* Displacement is also a vector quantity, representing the straight-line distance and direction between the starting and ending points.

Therefore, the magnitude of average velocity, which is essentially the length of the displacement vector divided by time, represents the average speed of the object over the given time interval.

Important Note: While the magnitude of average velocity is the average speed, the average speed itself might be different from the average of the instantaneous speeds. This is because average speed considers the total distance traveled, which might be longer than the displacement if the object changes direction.