The language that your mother and people around you speak gets heard a lot and then automatically goes to the brain where it is registered as the main language that a person speaks. When learning another language, the language is not spoken all the time around you so the brain doesn't pick the language up as quickly so it's a bit harder and more effort to learn that language.
I feel like learning vocabulary is more important in order for people to understand you, although I think grammar is important as well.
Of course it is possible if your writing and reading skills are much better than speaking skills.
Well, since English is my foreign language, it is more than beneficial as it is universal. In addition, knowing Spanish as another foreign language is really useful when travelling around Europe to countries like Italy, France, Portugal etc. because people don't tend to know English and Spanish is very similar to their languages.
Learning languages broadens your mind and helps you understand cultures.