As we can see from 'Find the balance point' the lower third of the bow is much heavier than the upper third of the bow. Our fingers on the bow hold are also uneven; the first finger is strong and can exert substantial downward force while the 4th finger only has a role supporting the bow in the air or while playing in the lower third of the bow.
This finger interchange exercise ensures that:
in the lower 3rd of the bow the 1st finger does not press. Here the 4th finger is supporting the bow
in the upper 3rd of the bow the 4th finger does not press, this could lever the bow into the air. Here the 1st finger (and also the 2nd and 3rd finger at times) makes sure that the bow hair is firm enough into the string
Find the Balance Point