I’ve recently had a requirement to rebuild a clunky and inefficient Android App with only partial access to the codebase (layouts and assets, but no Java/Android code). One of the issues was the App was talking to an online database via HTTP POST and GET. I had details of the HTTP root URLs used, but not the specifics on the parameters or how it was formatted.
So after a bit of digging, I found Drony (Google Play link), a kind of log watcher that can show you the exact nature of the HTTP requests, and from this tool I was able to see the exact format of the keys and values for the parameters to upload and download the right information. Great tool, really saved me a lot of effort (and stopped me having to chase original developers etc.)!