Nginx is built as a reverse proxy, useful to intercept traffic coming in to a server and route it to the correct service. This is the opposite, I guess we can call it an obverse proxy: it sits near a client and allows the owner (or a malicious party) of the client to intercept the traffic as it leaves the client. A former employer of mine used something like this to gain access to the API calls being made from iPhone apps, but you could use it to say attach authentication to calls heading a certain way (think what AWS is doing to make service to service calls work transparently to the user) or tons of other stuff (blocking content, if you’re so inclined).
I haven’t tried to use nginx as an MITM proxy or this project at all, but presumably it’s easier to use this when your usecase lies closer to the client than the server and vice versa.
Tools like Fiddler allow traffic modification based on rules, so you can tamper traffic. Forwarder doesn't appear to allow modification (other than headers?) so I think it's about logging traffic and collecting metrics. Still looking at the docs, I could have missed something.
I haven’t tried to use nginx as an MITM proxy or this project at all, but presumably it’s easier to use this when your usecase lies closer to the client than the server and vice versa.