Charles Proxy is a GUI to "man-in-the-middle" (MITM) yourself by snooping and/or modifying your own HTTP(S) requests. Usually, this is seen as a problem. However, it can be an immensely powerful tool for front-end development. I will explain what can be done with it.
Unfortunately, a production environment sometimes includes data which is missing within development or UAT environments. Perhaps an edge-case of the system is the reason. If I need to test a front-end change with this set of data, I could use the "Rewrite" feature of Charles to replace the body of a development or UAT request to be whatever data that I need to test. This often times is much faster to do, rather than mocking up the data for the interface to use.
In the last example, let's say you spun up a resource in Europe (you know because of privacy laws) for your backend. You want to test end-to-end that the new region is working, before pointing your library to this new region. In this case, you can "Map Remote". Just swap your US region DNS with your EU region DNS.
Hopefully this illustrates a valuable tool for you. Sometimes working with the front-end of the web can feel like having super-powers. You can do anything you want and most likely no one will know. The same cannot always be said when you make changes to backend technologies.