What are the benefits of using semantic HTML?

Clear Code is More Maintainable

Besides benefits to the user, semantic HTML makes more sense to humans: you and your colleagues.

If you were coding JavaScript, would you write functions like this?

dothething(q);
CmtFn(x);

...or like this?

loadUserData(username);
parseComments(language);

If you said #2 is better, well, the same principle applies to HTML. Labeling things clearly, indenting code properly, etc makes your code easier to maintain.