One of the benefits of using semantic HTML is that your site becomes more accessible to users with visual disabilities. This is because of the fact that screen reader applications make some assumptions about the content of your site depending on what tags you use in your markup. These assumptions are based on the semantic meaning of the HTML tags, which is why semantic markup fits screen readers like a glove.
Writing semantic markup is not always that easy, though. The choice of how to mark up the content of your site can be difficult. To check the accessibility of your site and make these choices easier you can run your site through a screen reader and see which markup makes the most sense.
An excellent way to see how a screen reader would present the contents of your site is to use the Firefox extension Fangs. This extension can generate the text that would be read by the screen reader Jaws.
When you have installed Fangs, all you have to do is right click and choose "View Fangs", and tada, there is the screen reader output in plain text.