When you are working with ajax, the problem is that after you have loaded some content using ajax, you can’t change the URL of the browser according to the content. Because of this, reloading the page causes the new ajax content to disappear and it shows the previous page. Although you can resolve this problem with having some hash tag in the URL, but having hash tag in the url for navigation won’t be SEO friendly.
Do you ever wonder when you are working Facebook or Github in a HTML5 supported browser, when you click on the links, the content is loaded into the page using ajax and at the same time the URL changes in the browser according to the specific page but without hash tag in the URL.
This makes use of the HTML5 History API to change the browser URL without refreshing the page.