AJAX updates section of page contents, without reloading the entire page. This feature is essential when user needs fast results, without any delay of waiting for the request sent by clicking the submit button and waiting for the response. The server load can be reduced by applying AJAX with less number of requests which saves bandwidth.
You can use AJAX for form validation. Instead of pop-up windows, the lightbox can be used, sorting and filter the content in tabular format, displaying results or votes using the chat application, reloading captcha for authentication checks.