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).