It can make sense to have duplicate communities on multiple instances in some situations. For example, if instance A and instance B both have Technology communities, but instance A is defederated from instance C, then that redundancy is valuable to instance C.
There are also ways to reduce redundancy — for example, different rulesets on instance A and B could result in different content in their respective Technology communities. And the questions that show up on each are bound to diverge in some ways.
Ultimately, though, my hope is that admins on the post ranking flank of the fediverse starting looking for more ways to distinguish their instances from one another. Self-hosting presents a number of opportunities that weren't really available on a siloed corporate platform like Reddit. There's no reason, for example, that an admin couldn't start up a Medical instance, and subdivide it into much more topical communities/magazines than you'd find on a "generalistic" instance, e.g. Neurology, Cardiovascular, Podiatry, and so on.
(Just as an aside, Beehaw is a Lemmy instance, so it's not really distinct from Lemmy in the same way that Kbin is distinct from Lemmy.)