Using it every day at work as a simple reverse proxy with debugging capabilities. Any call my (local) frontend does to the backend goes through mitmproxy so I can quickly figure out what's wrong, edit and replay calls. Production env uses more scalable tools but mitmproxy does a formidable job for development environments with almost zero setup costs :)

I use it when pentesting/bug hunting to examine the way the application works and to modify traffic in order to break stuff (either the app or the server-side API it uses).

Basically the same things you would use Burp for, except mitmproxy has two important advantages:

1. Supports a SOCKS5 mode and has much better support for intercepting non-HTTP TCP traffic.

2. Easier to extend quickly via Python scripts.

Some frameworks make it surprisingly difficult to see raw, unaltered traffic. I have used tools similar to mitmproxy to debug HTTPS traffic for Apache CXF because the framework's built in logging is kind of useless for debugging errors.

Reverse engineering / sniffing HTTPS traffic to figure out what API calls an app makes.