Because writing reflects thinking, if your thinking is obscure and convoluted your prose will be, too.
Complication and obscurity often reflect insecurity on the part of the writer or speaker.
In practice, self-described ‘purists’ are often ill informed about what actually constitutes good usage.
Students don't need to have their own faddish or unthinking linguistic habits merely validated at school. They need to have their communication skills sharpened and elevated, lest they enter the adult speech-world handicapped by sounding ill-educated.