Grammar

I believe the most important are:

To be able to say things in the way that you mean them.

To be able to understand things the way that they are meant.

To help you to explain things and make the words a little easier to put together.

It's bit like reading a map, or doing a jigsaw puzzle.

Yes, it's true that you learned to speak your own language before you knew any grammar, but
it is also true that people were trying harder to understand what you were saying, because they knew that you were just learning.

While it isn't very important at this stage to learn advanced grammar, you do certainly need to know the basics.

I suggest starting with a children's grammar book. This simple beginners book is a good one... Basic English Grammar: For English Language Learners: Book 1 by Saddleback Educational Publishing.

Next, you could progress to a much more comprehensive book with exercises and answers:
English Grammar In Use with Answers and CD ROM: A Self-study Reference and Practice Book for Intermediate Students of English by Raymond Murphy

"Why is Grammar important ?"

Because words without grammar have little meaning!

Grammar is necessary for a number of reasons...

Grammar will help you study the structure of the language,

to learn how to put your sentences together correctly,

and to speak the language well enough to be understood perfectly.