2) Bad writing makes you look bad
Not everyone, when presented with poor writing, takes the time and trouble to wade through it. Many just conclude that the author is either careless or stupid and don’t waste their time reading it.
Can you blame them? How much time do you spend in clothes shops where the material looks cheap and the products are badly displayed? How often do you go back to a restaurant where the staff were rude and the food under-cooked?
You could argue that those are fatal flaws because it was a problem with the basic product or service. You could argue that, unless you’re a professional writer, writing isn’t essential. Why should a clothes retailer, a restaurateur, or an accountant, recruitment consultant, or website designer for that matter waste time getting their writing spot on?
For the same reasons that they invest in branding, marketing, or for that matter for the same reasons that they bother dressing smartly for work and talking coherently when they’re there. Writing is one of the ways we present ourselves to the world, and if we take no care of it, if we pay no attention to it, people will think less of us.