Question

Why is it useful to write a ratio of fractions as a unit rate

Answer 1

It is useful to write a ratio of fractions as a unit rate because it makes it easier to compare other unit rates to the corresponding unit rate.

Answer 2

A "unit rate" has a denominator of 1. That will often simplify any subsequent math operations.

_____

The choice of the form of a ratio should be made based on what you need to do with it. Sometimes, a denominator other than 1 is appropriate to follow-on operations you may need to perform.