Every developer knows how difficult it is to debug an application that doesn't want to work. Network applications are especially unpleasant to debug since the networking debugging tools are usually not convenient at all.
Charles comes as a solution to this problem: by setting up a proxy, Charles is able to view and modify any HTTP/HTTPS request that passes through it, allowing you to know exactly what your app sends and receives.
If your app works properly, you can also modify responses of your app with the help of breakpoints that enable you to check whether it behaves properly or not in case of malformed or invalid data. If you have a stunningly fast Internet connection, you might nonetheless want to check if your app works the same way with a slow bandwidth, and that is where the throttling tool of Charles proves its usefulness. With it, you can simulate any connection speed using predefined configuration profiles such as 56 kbps modem, 3G cellphone or by setting your own speeds.
To sum it all up, Charles is a great way, might even be the best one I've come across so far, to debug any app that performs HTTP and HTTPS requests. However, its price might discourage occasional developers from using it.