bbPress is a forum plugin for WordPress. Since most forums allow you to interact with them via email there are many people looking for this feature in bbPress. Out of the box bbPress does not have the ability to send or receive emails so many people look to Postie to help.
Unfortunately there are many difficulties in getting Postie to work with bbPress (or any other forum software) due to the fact that the incoming and outgoing emails need to have some way to hook them together so the right forum topic is updated. Postie only deals with incoming emails so it is not an ideal solution for this problem.
Luckily there is a ppPress plugin located here: https://github.com/rmccue/Falcon
There are some limitations, but it seems to be the only solution that supports a full round trip emails.