Any formatting in email is html.

There is nothing that requires any text to have any formatting. But basic formatting can help readability. There is a reason to include bold or italics. Or headings. Or maybe even a simple diagram/image, or make links more approachable than the raw url.

Snail mail letters can include all of the above (except links), why not email?

Even when written memos were the norm in business, they still had company letter head.