I used to know someone wealthy whose continued wealth relied on working with local and state governments. This person's public correspondence in lawsuits and with local government officials was purposefully littered with spelling, punctuation, grammar, and capitalization errors. When I asked them about it, their response was that it was on purpose so that they seemed less smart and thus less threatening, with the hope that they would get more favorable rulings and contracts by not seeming like "one of the big entities."

I'm not asking you to believe me on this, but sharing it more as an anecdote of: something on the surface is sometimes not the reality of what's underneath.

In addition, it broadcasts that the sender is too busy with all their important work to spend time refining and proofreading, that you're getting their raw, unfiltered thoughts directly from them, not through an assistant, and that their time is more valuable than yours so the burden is on you to parse their stream of consciousness jumble for precious nuggets of their exclusive wisdom. The semiotics make sense, plus it's just easier and faster.

I remember being told that many of the spelling/grammar mistakes in (English) menus for ethnic restaurants were deliberate to make the (English native speaking) customers feel superior.

(Also not saying I believe this at all, just relating an anecdote).