By using mitm, basically "pretending" you're the site the victim wants to connect to and trasparently connecting to the actual upstream site. Basically decrypting the traffic locally for inspection before sending it back out. https://en.wikipedia.org/wiki/Man-in-the-middle_attack. You don't need a root CA, you just need to poison the DNS to point to the mitm server and just present any old valid cert for the domain so it doesn't trigger a self-signed warning or whatever.