How much does knowledge of linguistics help with learning foreign languages?

[Aoi Magi]

As far as I know, it varies from person to person, but our human brain works through association so learning languages gets easier if you already know a language similar to it and an ancient root of the language you are trying to learn obviously will have a lot of similarities.

As for knowing those rules, I personally don't find them very useful. While using a language in practice we never think of the actual rules rather we just say whatever sounds right to the ear, and for me just practising with a native speaker without bothering about grammar and such seem a lot more useful.