So, I pay for content by allowing someone to annoy me?

But the last time I cheked, none of the markdown-like systems offered that in a reasonable way. Semantic markup means that I do not simply make some text bold or italic, but I give the reason why it should be somehow formatted differently than the surrounding text. Reasons could be that it should be emphasized, it introduces a new term, or it is a foreign phrase (such as "a priori"). All this is usually set in italics, but sometimes it makes sense to change this for certain categories. Also, I guess, alternative output devices (such as audio or Braille devices) could benefit from this, although I am not aware of any practical details.