In writing that is meant to be referenced again and again, good grammar matters and shows care was taken in the writing. Think of a doctor or scientist. We expect them to have a well thought out position, that is clear. In contrast, just as an improperly configured telephone wire can cause static during a phone conversation, improper grammar can likewise affect the meaning and clarity of an intended message. Writers should be professional by having correct grammar. Some common errors are with sentence structure, subject/verb tense, punctuation, spelling, and other basic mechanics and parts of speech. Even something as simple as a misplaced comma can completely change the meaning of a sentence. For example: "Let's eat Grandpa." vs. "Let's eat, Grandpa." Grammar makes written content more readable and in turn more interesting. If it is necessary to repeatedly reword sentences while reading, the flow becomes disrupted and involvement in the story halted. Therefore, grammar lays the groundwork for effective communication.
This is not to say poor grammar is unreliable or not well thought out. It’s a courtesy to the reader. There are plenty of writing checking tools that can help polish writing.
Rejecting the premises
Good grammar could be used by bad actors to make a poor argument seem more official.