In general the advantage of having two softwares sharing the userbase is terribly useful when you have community generated content. You don't force them to create a new account and if is well integrated you can share the bio, profile pic and everything, making it smooth. Specifically with the CMSs, some use them as kind of "social network" (for example with buddypress), or a forum can be a place to have internal members discussions, which as soon as it gets over 10 members can be difficult.
In general you want to make it as easy as possible for contributors to contribute, a forum can be a a perfect place to solve problems, post internal guides (which WP doesn't have as a feature) and so on.
Can't wait for flarum to be stable and see all those integrations popping up!