You don't need Ajax but it might make your web user's experience a little better.
With any server-side language, when you want to show new data, you have to refresh the whole page. The page reloads and the interface changes.
This doesn't happen in an Ajax application. You can use Ajax to gather data and display it without reloading the page or changing the interface. That gives your user an experience closer to that within desktop applications and might make their web browsing more pleasant and easier.
subject: Beginning XML with DOM and Ajax: From Novice to Professional