Honestly, like all decisions, there were many factors. A big hurdle in this instance was that the existing application was written using a PHP framework that made generating something like a SwaggerDoc an extreme pain. Since we knew we'd have to write documentation outside the product anyway, and we were already using Postman for tests, using their (free, and already hosted someplace) docs was a no-brainer. It even allowed us to share the docs with 3rd party developers in a way where they could import the docs locally and run them -- again, via Postman in order to test their own uses of our API (simply by setting up a few variables like putting in their API key).

From point 1 it sounds like they already use Postman for testing and so using it for documentation makes it easier - perhaps its somewhat automated.