Proper grammar leads to effective communication. Without it, one may be able to get a meaning across, but there are ample opportunities for miscommunication or misunderstandings to occur. Accuracy of communication, internalization of grammar rules, and excellent first impressions are the key benefits of learning and using proper grammar. If one wishes to be fully understood, one must harness a deep understanding of grammar rules and conventions in speaking and writing. Accuracy is important in communication because without it, one may be misheard or misinterpreted, and issues can arise from the miscommunication. This is especially true in the workplace, where directions must be given clearly in order for things to run smoothly and for productivity to increase. It is also true of interviews, meetings, or presentations in which one must make a good first impression in order to be taken seriously. Internalization of grammar rules means that one can become more fluent in the language that they speak. Language and grammar are things which must be learned. We do learn basic communication by listening and repeating from the time we are infants, but to learn fluently we must go to school and study grammar. Fluency leads to more accuracy, better first impressions, and better communication overall.
People know intuitively how to communicate without knowledge of grammar. As long as one conveys the meaning they intend to, then the grammar itself is unimportant. People can learn to avoid miscommunication by paying attention to the conversation and thinking carefully before they speak. Fluency is attained through speaking and writing throughout one's life. It does not need to be taught; it only needs to be modeled to be internalized by language learners.
[P1] Knowing grammar is necessary for accurate communication. [P2] Language and grammar must be taught.
Rejecting the premises
[Rejecting P1] Knowing grammar is unnecessary for accurate communication. [Rejecting P2] Language and grammar are learned intuitively and do not need to be taught.