Users may be managing multiple Domino domains for various reasons.
One thing that users in such environments are struggling with is searching. The current site search supports only applications in the domain. In an environment where there are multiple domains, it is necessary to use the site search for each domain, which is troublesome. I imagine that this restriction occurs because it is necessary to copy catalog.nsf to another domain, but sharing catalog.nsf between domains is one of the challenges in this environment.