Before AJAX existed, when the massive Internet started to grow all websites were constructed as a set of multiple complete HTML pages and whenever an action was performed by the user, this action requested a complete page to be loaded from the server, this was proved to be inefficient both in data transfer and user experience.

Since you had to load a complete page in order to perform perhaps a small change in the webpage this generated large amount of data to be transferred to accomplish this. And from the user experience perspective, the end used most of the times had to see the webpage goes blank and then loaded again.