Charles lets you select a request and repeat it. Charles resends the request to the server and shows you the response as a new request. This is useful if you are making back-end changes and want to test them without having to repeat the request in the browser (or other client). Especially if recreating the request takes a bit of effort, such as achieving a score in a game, this will save a lot of effort.
The repeating of the request is done inside Charles so you can't view the response in the browser, or other client; the response is only visible in Charles.