If we don’t have an academy to regulate the English language, why are there even grammar rules? The best answer I have found is that language’s only purpose is to communicate clearly and effectively with others. One way to be clear is to use mutually-agreed upon conventions. Yoda may be able to get away with “Careful you must be,” but “You must be careful” is more natural for most of us to understand.
We speak and write differently depending on who is listening or reading what we say. When speaking with young children, we use simple sentences and words; we use informal language when talking to our friends. However, some basic conventions help us understand each other. Rules of grammar, including standard spelling, capitalization, punctuation, and word usage are just ways to get our ideas across.
Standard English includes spelling, pronunciation, syntax (the way we put words together), and vocabulary. Using a form of “nonstandard” English is only useful if the people you are speaking with or writing to are also familiar with that dialect. One way that each young generation separates itself from the older ones is through the vocabulary they invent or repurpose. As they grow older and their slang becomes commonly used, the next generation of teens comes up with its own slang.