Trace / view http header data, see cookies being sent and received - control whether they get sent or not. See what the JavaScript on a web page is actually doing. Browse anonymously by withholding your Referer tag, and user agent. Even change your Referer and User-Agent tags to pose as someone else. Find out what sort of URL redirection the site may be using. A must for webmasters, or as a debugger for any one writing software that uses http.