Intercepting Requests to Unlock Hidden Functionality
When you fill out a form on a website and hit submit, your browser constructs a POST request and sends it to the website so that it can decide what to do with that information. It would contain not only the data you entered into the form but information about your login session and a cross-site request forgery token (hopefully).
If you're interested in understanding a website's functionality, capturing requests as they exit your browser lets you examine how to effectively interact with the site, as these requests are crafted by developers to elicit successful responses. You can use this insight to create custom requests, which can sometimes yield intriguing results.
Let's look at how to do that with a simple example.