I relayed a story to my team this morning on this very subject, related to the general level of quality - or lack thereof - in some recent documentation that had been created.
For me, proper spelling and grammar are paramount. Just as it only takes a few seconds when you walk into a room for most people to form a positive or negative first impression of you, it only takes a few sentences of hearing you speak or - especially - reading something you've written.
When I'm writing - regardless of whether that medium is an email, a text message, a post on an online forum, or whatever - I strive to ensure that I represent myself and my intelligence appropriately, rather than doing myself a disservice and/or disrespecting others by demonstrating a lack of concern. I've always encouraged my coworkers and, especially, my subordinates to pay extra attention to their spelling and grammar when writing anything - or at least anything work-related.
When you use proper spelling, grammar, punctuation, sentence structure, paragraph breaks, etc., the natural reaction by others is to perceive you as being more intelligent, more educated, and more authoritative. This is exemplified in that aforementioned story...
I once had three juniors working under me who were all hired within the same month. The first was quite talented, the most talented of the three, but was a bit undisciplined. The second was less talented, though still above average, and had a better work ethic but was a bit headstrong. The third - let's call them Sam - was less talented, had only an average work ethic, and had a less relevant degree than the other two but was very coachable and eager to learn. Given their differences, they actually worked quite well together, as a team. After less than a year, it was decided - by people outside our department - that one of them would be promoted to a cross-functional team with an elevated role. All three were eligible, being considered, and hopeful for this promotion.
To spoil the surprise, Sam got the promotion.
Despite being less talented, having a less relevant degree, and even having fewer merged pull requests (a primary metric for our department) than either of the other two - Sam was picked above them for the promotion. The reason is because Sam was the only one of the three to take my advice about written communication to heart - going out of their way to use the tools available to make sure they represented themselves to the best of their ability in their writing. The other two considered their written communication to be "good enough"...
The team lead that picked Sam for this promotion specifically pointed out the quality of Sam's written communication - chats, emails, and especially documentation - as a primary reason for choosing Sam. To them, Sam seemed to be the most professional, intelligent, and authoritative candidate.
As communication has gotten easier and more commonplace in our society, an overly casual nature has also creeped into that arena. For those of us seeking an elevated style, it's important to remember all aspects of style - including our communication.
People may most notice your clothing when you walk into a room. When you leave that room, however, people most remember how you communicated yourself.
As I'm finishing this post, I'm having a feeling of deja vu - as though I've previously shared this sentiment. Perhaps I have shared it in this community or perhaps it was elsewhere. Regardless, I feel it is a principle worth reiterating...