A message is not less important or worthwhile if it contains spelling or grammar mistakes.
A person is not less valuable or worthwhile if they make spelling or grammar mistakes (corrects the third fourth fifth typo).
It is utterly important that people who find spelling or grammar more tricky, either because of a specific learning difficulty such as dyslexia, going to school during the period when grammar was not explicitly taught, or because it is just quite tricky sometimes, are supported.
A lot of the time, we learn from what we see. Of course it doesn't matter on a forum if we write "would of" instead of "would have". But it matters enormously in a job application, and if we are used to seeing "would of" on the forum we read all the time, often, that is what will stick in our heads and make it on to the paper.