The value of using the International System of Units when describing motion is that these units are universally accepted - so an observer that uses these units to describe a motion in a place can compare his results to the results of the same experiment made by another obserber in a different place.
For example, if I measure the speed of a car, and I measure it is 20 m/s, and then another observer measures the speed of another car, and he finds it is 25 m/s, we are able to compare the two speeds and tell which one is greater. On the contrary, if we use two different units (e.g. km/h and miles/h), it would be difficult to compare the two results.