Ain’t grammar just a bunch o’ arbitrary rules nobody never needs nohow?
Most people feel that grammar is filled with rules that change from sentence to sentence and paragraph to paragraph. There is a view that as long as one can basically communicate with the average person, there's no need to master this grammar business.
This is a mistaken view.
Grammar is the fundamental subject upon which every other subject builds. For hundreds of years, the first several grades a child attended was called “grammar-school.”
This was done so for two critical reasons.
First reason: We think in words and sentences
The way to look at this is that when you think about anything, whether how to cook a meal, or how to present a complex new business operations system at your company, you are contemplating in words and sentences.
For example, to even begin to cook a meal, you have to think about what product you want (the finished meal), and how you will create that product (the ingredients, measurements etc). This process is based on your previous knowledge, which is all accomplished through thinking by use of language.
Language has two parts: individual words and the relationship of words to communicate thought. Grammar deals with the latter (vocabulary deals with the former). A formal definition for grammar: it is the science, which studies the relationship of words to communicate a complete thought.
And of course the whole field studies what is a complete thought, and how words are used to convey one's thought. This includes the eight parts of speech, which means studying what exactly is a noun and what exactly is a predicate (verb) among all the rest. It also studies things like diction, euphony, economy of words and more.
Second Reason: We communicate our thoughts through words and sentences
Now, Listen to the CEO of your company, more than likely their grammar is quite elegant—assuming they're any good at their job. If your CEO isn't a good example, look at CEOs of top organizations in any field. For example, John Allison, the former CEO of BB&T, who took that company from $4.5 Billion in assets to $152 Billion, is a great thinker and speaker. Listen to him speak here (https://www.youtube.com/watch?v=VKw8VhV0N1E). Or watch a video of Steve Jobs, Warren Buffet, or just about any successful person.
You'll notice that when they speak or write, their thoughts are clear and concise. Their meaning is easy to understand, with each thought logically flowing from the preceding. It is not a coincidence that successful people tend to be proficient in grammar, because this logical continuity and clarity of meaning is exactly what the study of grammar entails.