Java applications can help make more efficient use of bandwidth, for exaple by providing form validation at the client. Unfortunatly Java applications are often used to add novelty to home pages, without providing any additional usefull information.
While Java applications can make web pages more spectacular and fun to look at, they can also be yet another item to download over a slow link with limited bandwidth. It can be very frustrating waiting for Java applications to download while visiting a site to get useful information.
For more information about Java, visit SUNs Java ® page.