What’s the point of having grammar rules when language constantly evolves?

Answer 1

If you're a native speaker of a language, possessing a thorough understanding of grammar isn't really necessary to be conversant. For a language like Latin, however, which no longer has native speakers, it's helpful to have a shared vocabulary to describe how the language is functioning. It facilitates the acquisition of that language. Also, the pace at which languages evolve isn't overnight, doubly so with Latin, which is more or less a "dead" language.