Because mere humans like to have images, colors and text formating in emails.

Because nothing else has anything beyond the most minimal support for right-to-left languages.

Aside: it's nice that the site checks <bdi>, but dir/direction is much more widely used in practice and should be added.

It's convenient for underline, bold or italics in some long winded messages.

That's why we have lightweight markup syntaxes like markdown.

Markdown is a lightweight syntax for writing and generating HTML, like BBCode. It's not an alternative syntax to HTML.

Markdown is designed to be readable without encoding it as HTML though, so is nicely readable on old email clients too.

The idea is that it gives a standard for semantic markup that can be read as plain text. Lists, highlighted text, block quotes, etc. that is all plain text but easily parsed by humans.

I want to write mathematics in my emails all the time, and sometimes I want to communicate with people who don't yet know LaTeX.

HTML emails are a blessing. Receiving them allows me the ability to quickly glance at an email from an unexpected sender and immediately recognize it as a marketing email or cold sales, and send it to the spam folder without needing to read a single word.

Every reason listed in this subthread, but also tracking gifs and other means of spying on the end user.

Email clients block remote images by default.

If an email client does not, it's a bug.