Knowing Grammar helps us to easier in communicating. For example, imagine you have a job interview, you need to talk about your past job experience. We apply the rules for Present Perfect tense. Because you talk about experience.

When you meet a new person, you talk with them about yourself, you apply the present simple tense to talk about facts and regular habits. It is clearly that the verbal clauses and adjective clauses help us enrich our sentences and add more information. Meanwhile, grammar rules help you to make your communication to appear coherent and more logic.

English speakers tend to speak very fast with a lot of short forms and silent sounds. Knowing grammar helps you to catch these short forms and silent sounds by guessing the idea what the speak want to say.