As an example: If you have an account on kbin.social you can follow both !fediverse@lemmy.world and !fediverse@lemmy.ml, but these communities stay separate. This often leads to duplicate posts, and splintered communities. What NodeBB and Discourse have done is equivalent to if !fediverse@lemmy.world and !fediverse@lemmy.ml could follow each other, so a post in one of the communities would show up in the other community.
That's a neat feature! I think community aggregation in this sort of way is a positive and it could be useful for lemmy as well.