Question

Why is writing rational numbers as a fraction of as decimal useful?

Answer 1

Rational Numbers won't be confused into getting to Integers, Whole Numbers, And Counting Numbers. 

Like ALL Counting numbers go into whole numbers, ALL whole numbers go into integers, and ALL integers can go into Rational numbers.

If you go the other way around not ALL can go 

Answer 2

Fractions are more accurate than decimals. This prevents repeating decimals.