This is the official documentation for Converse.js. If you’d like to contribute, please read the Writing Documentation page.
You might instead be looking for the User Manual.
The benefit of using converse.js as opposed to relying on a SaaS (software-as-a-service) solution, is that your users can have a much more customized, streamlined and integrated webchat experience and that you have control over the data. The latter being a requirement for many sites dealing with sensitive information.
You’ll likely want users who are authenticated on your website, to also automatically be authenticated on the XMPP server. In order to do this, you’ll have to add some server-side code for your website.