One of the key aspects of it is the fact that it's external to the
browser, so you can be absolutely certain that you're seeing things
free of anything that happens _within_ the browser.
Features I frequently use, apart from the obvious examination of
request/response cycles, headers, structure of response data (it has
nice structured viewers for XML and JSON among others):
Rewrite: automatically modify requests and/or responses, whether
headers or content, according to a specified set of rules (including
RegExp matching/replacing);
Map Remote: specify (with wildcards if desired) that requests to some
locations should be transparently changed to requests to some other
locations;
Map Local: specify (wildcards again) that requests to some locations
should be served from local files instead - extremely useful for,
amongst other things, debugging a live site that has minified JS
files: simply map them to a local copy of the debug version, insert
debug code in there, etc. without changing what's on the server;
Mirror: save all responses from a location locally;
Block cookies: useful for debugging cookie problems without having to
actually clear your cookies;
Blacklist: useful for seeing what happens to your Ajax code if the
server becomes unreachable, without having to actually take the server
down;
Throttling: see how well your site works over a dialup modem, without
dropping your broadband connection;
Breakpoints: intercept and manually edit requests and responses
according to your chosen criteria, without the browser ever realising;
... and there are various other things it can do, as well as uses for
the ones I've listed that I haven't even thought of yet :-)
Regards,
Nick.
--
Nick Fitzsimons
http://www.nickfitz.co.uk/