You can use markdown as source for formatting, but the source is very readable by itself.
This is extremely important for writing and also for fall back purposes. Any time there is any kind of a problem in writing or reading or interpreting, just fall back to source.
People can say html is human readable but the difference is huge. I didn't even know that markdown is parsed on hacker news, I just used stars as underscores as natural formatting.
There's also potentially much much less overhead. Ultimately, browsers and your cell phone apps etc should just format markdown natively, meaning huge performance boosts, as html, css, js etc has just become so complex for many tasks as a consequence of do-everything.
My phone's email client struggles browsing back and forth of what should be just a few small text files and file attachments. It uses way too many cycles since it's also a jpeg and css and js renderer with scaling and everything.
As someone who's used text email and news groups, lightweight formatting is very natural and informative. Those services would not have been possible in the past if they had been bogged down with large amounts of excess complexity.
I guess reducing overhead is not sexy because it doesn't involve new features. In the future everybody will just create their typed static oneliner email as 10000x10000x10000 pixel 100 fps 3D movies of 2 minute length (you see, that's the time it takes to read it), and if some supposedly slightly technically literate person asks that perhaps they should use a word 2020 .doc instead for reduced file size, the answer is "but you can't do a space station flythrough in it".