Reading helps you to improve your vocabulary and get know new grammar constructions, collocations. All these new for you things you can use in writing and speech. Of course, you won't see an improvement without practice.

If you read a lot of books, if you think about it more, for example, you think about how to improve you English all the time and at the same time you read more books. Maybe it is helpful.

It depends on if it is formal or informal language