Let's say your servers can broadcast to the following network range:
And you have clusters in the following networks:
You launch a new server set up to join by broadcasting, it will broadcast to 10.0.0.0/16.
Which means you have absolutely no idea which cluster it will join. Because any of the three clusters in those networks will accept the request.
Now you have a cluster configured to unicast to a node in the cluster seating in the 10.0.1.0/24, let's say 10.0.0.1.
Now you know exactly which cluster it will join.
Same applies if you have 3 different cluster in the same network range.
if you have one cluster only in one network range only, this may not make a difference now. But may cause problems if someone creates another cluster with broadcasting in the future.
Does that make sense?