As an example: If you have an account on kbin.social you can follow both [email protected] and [email protected], but these communities stay separate. This often leads to duplicate posts, and splintered communities. What NodeBB and Discourse have done is equivalent to if [email protected] and [email protected] 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.