It is possible that you'd like to have a Functional Location structure hierarchy built at a higher level, for example to group Plants by business line, or by geographical location, for example. In such a case, you might have FLs at the Plant level in the hierarchy, but their superior Locations are not for the same plant. And to segregate authorizations, you might add them to another Maintenance Plant, instead of adding Authorization Groups, to ensure that PM Orders are not created above the Plant level.
This is one example, off the top of my head, of combining different Maintenance Plants and maybe different Structure Indicators in a Hierarchy.
Can you also see the thread: Functional Location Hierarchy Mapping