Two examples of websites that rely heavily on AJAX are Google’s Gmail (www.gmail.com) and Facebook (www.facebook.com). Without AJAX, it wouldn’t be possible for Gmail to update your inbox at the rate it does and Facebook wouldn’t be able to list completions to your friend search before you hit search.
Load time isn’t the only thing you save by not having to reload the entire page every time a users wants to interact, it also saves you bandwidth. If your website was created with AJAX in mind the reduction in bandwidth usage compared to the same site without AJAX support would be significant. There are no figures to offer on actual bandwidth saved as every website is different. However, if a web page that is five kilobytes is reloading itself every time a user has to send one kilobyte of information, and the page that is reloaded is the same with the addition of just a success or failure message based on the users input, you would be saving four kilobytes of bandwidth every single time a user enters said information. If you have a thousand users that all use that page a single time using AJAX would save you roughly four megabytes of bandwidth and save your users precious time.