In Creo, the assembly tree (CAD product structure) is necessary the same as the file structure (.asm and .prt)
Ability like in some other CAD tools (CADDS, Catia v5 ...), to define in assembly tree some logical nodes with "normal embeded features" like positionning constraints, assembly features, etc ... (ie not only a group)
So being able to build an assembly tree different than the file structure (like in the second example). And being able to configure in creo Workgroupmanager if the CADoc structure is in synch with CAD File structure or CAD product structure
It is usefull , notably when using Windchill CAD integration and associated WTpart xBOMs (Design, and downstream Manufactruing or Customer Service BOMs).
The CAD designer can create some assembly levels that we do not necessary want in Wtpart design BOM
A special WTpart type exists for this case. which can be use to handle these "pure CAD levels assemblies" .
But as they are not automatically managed in Windchill (except in BOM reporting) all downstream Users have overloaded business tasks to remove them from Manuf and CS BOMs, manage quantities (notably sum different occurences of the same component used in different CAD levels)