Proper nussach is the mesorah for the way a particular tefilah should be recited.
The best example would be Kol Nidre, which thankfully, most people still know.
But the proper nussach for many of the other tefilos on Yomim Nora’im is being diluted (and convoluted)more and more as the years go by.
Many of these tunes go back hundreds of years.