Re: What does the one-click bbPress forum integration do exactly?
There is no conversion, and there is no import.
With versions of BuddyPress previous to 1.1, bbPress was an external library, meaning it was integrated with an XMLRPC bridge but was never included in the core of BuddyPress. This meant that bbPress could technically be anywhere and BuddyPress could still get at it, but it also meant that it was more difficult to integrate and make play nicely together.
Only use the existing bbPress installation option if you used a version of BP older than 1.1, or you had a separate installation of bbPress that still shared the user tables with an existing WPMU/BuddyPress installation.
Check the forums codex entry for more info: https://codex.buddypress.org/developer-discussions/buddypress-forum/