Here is where I sound like your fourth grade teacher. Grammar IS important. But it’s not rocket science. You can learn all those pesky rules (and English has more rules than most languages).
Why? Because knowing the rules will make you a better writer. When you know proper grammar, it’s easier to eliminate dangling participles, incorrect possessive pronouns, or unruly adverbs.
Bad grammar stops the reader and takes them out of the story. They may not know why, but they know something isn’t flowing. And if it happens too many times, they may stop reading altogether.