Almost completely pure way of storing ideas. With this I mean that you don’t store unnecessary data such as “background should be white” or “left page margin is 1.3cm”. It’s just text. What’s important is what it says + minimal markup.
Presentation is left to the reader’s client. Do you want dark mode? Get a markdown editor/reader that supports it. Do you want serif font? Again, that’s client’s choice and not part of the document.
I wish browsers would support markdown out of the box, so you could open example.com/some-post.md