As an author, an editor, a publisher, and a retired English teacher, the number of errors (not just typos, we all have those) found in posts by "authors" on social sites makes my teeth clinch in agony.
There is used in place of their or they're, or one of the others is used incorrectly. It's and its are interchanged incorrectly, as are your and you're. Those examples are simply a few of the grammar mistakes I read every day.
All right (and that's another mistake, using alright for all right), some people don't know grammar well, but writers definitely should. I don't know that I would want to read a book by someone who can't manage to understand the difference between homonyms and/or what version of a pronoun is used as the object of a preposition. For example, I often hear (hear not here), "That's important to Mary and I." Really? He would say, "That's important to I"? Actually, that is what he did say. A compound object is the same form pronoun as a singular object. And I have heard and read that problem from so called well-educated people.
I know some people think that using correct grammar isn't important, not even for writers. After all, editors are responsible for making a writer's work "pretty." Wrong. Many publishers do not edit. Whatever the author wrote, the way he wrote it, goes in print, if accepted.
Not only are writers expected to use their own language correctly, but businesses are expecting their candidates for employment to use grammar the right way. "Experts" say the use of correct grammar shows intelligence, creativity, and a desire to communicate effectively: traits that employers want in those hired.
I fought high school students for nearly thirty years, trying to help them understand and use their language correctly. I cringe when I read what some of them write on social sites because I know they should know better. However, I shudder more when people who call themselves authors slaughter grammar.
As I said, we all make typos, especially when auto-correct kidnaps our words, but correct grammar, punctuation, and sentence structure do matter.