Humans make mistakes. Grammar, typos, thinking errors like brain farts. You cannot really replicate those tiny human imperfections perfectly and they are what makes us what we are (yet).
Humans make grammatical, semantical, and punctuation errors, but a simple tool like Grammarly can detect all that and suggest what needs to be corrected. And there is nothing wrong with wanting your posts to sound and look good. Besides Grammarly, all popular browsers have spellchecks that help if you misspell something or make a typo. There is also nothing wrong in correcting such mistakes, so you don't sound like a 9-year old who can't spell. Couple the two tools together, and you get an almost perfect-like poster.