Here's why:

* Motion describes an object's change in position over time.

* Velocity is a vector quantity that describes both the speed (how fast an object is moving) and the direction of that motion.

Therefore, velocity fully captures the quantity of motion, providing information about how fast and in what direction an object is moving.

It's important to note that other related quantities are also used to describe motion, such as:

* Speed: Only describes the magnitude of how fast an object is moving.

* Displacement: Describes the overall change in position, regardless of the path taken.

* Acceleration: Describes the rate of change of velocity over time.

While these quantities are related to motion, velocity is the single quantity that encompasses the full measure of an object's motion.