The rules are a way of achieving a result that can be identified as belonging to a particular idiom. If you want to sound naively folky in a faux-archaic way, use strict parallel fifths. If you want to sound early-mediaeval, use a mixture of fifths and fourths and aim for Pythagorean intonation. If you want to sound Georgian, use stacks of strict fourths. If you want to sound like a Renaissance-to-Baroque choral group, use thirds and triads and try for meantone intonation. If you want to sound like something in the Romantic era, add chords of the seventh or the ninth resolving in the textbook ways. If you want to sound barbershop, resolve to dominant seventh chords instead and use just intonation.

Every idiom has its own stylistic rules. Big harmony books have lots of examples from different periods, they don't just say that one idiom is the true one.