An irreducible fraction (or fraction in lowest terms, simplest form or reduced fraction) is a fraction in which the numerator and denominator are integers that have no other common divisors than 1 (and -1, when negative numbers are considered).
What are the applications and uses of unreducible fractions?
A classical one is the proof that square root of 2 is irrational (https://en.wikipedia.org/wiki/Irreducible_fraction#Applications).
Are there other uses of it in Basic School (High School or earlier) besides to express a fraction in simplified terms as answer to a math problem?
Related question: when the concept should be first presented to students? K4? K5? Only in High School? Why?