> why we even needed the Avogadro constant / mole as a fundamental constant(...?)

It serves as a link between human-scale and atomic-scale observations, classically needed for chemistry performed on Earth by humans. This link must exist, as others mentioned, for stoichiometric calculations (e.g. air/gas mixture in a internal combustion engine). For much of modern scientific history, it was deemed useful to scale into an easily eye-visible human scale (the gram). [0]

[0] https://en.wikipedia.org/wiki/Avogadro_constant#General_role...