We need to be able to play in all parts of the bow; as the old joke goes - what is the point of buying a whole bow if you just use the top half?
Moving through the bow and stopping at various points is a good way to make sure that the bow-hand is flexible enough to support the bow while making a resonant sound. When stopping take as much time as you need to make sure that the bow feels stable in the string at every point.
Issues that make it difficult for students to feel free throughout the whole bow are already familiar to us:
The bow is heavier in the lower third than the upper third. See Find the balance point
It takes time to develop a flexible bow arm and bow hold so that you have enough control to add friction in the upper third of the bow but do not press in the lower third.
Sometimes players grip the bow tightly with the first finger, which locks the whole bow-hand, which in turn makes playing in the lower 3rd of the bow especially difficult.