Why keeping discipline models separate is what makes coordination actually work.
Open a coordination session in Navisworks or a linked Revit view and everything on screen looks like one seamless model — architecture, structure, mechanical, electrical and plumbing all sitting together in the same 3D space. That combined view is a federated model, and it's easy to assume that behind the scenes it must also be one combined file. It isn't, and that distinction is not a technicality — it's the entire reason multi-discipline coordination is workable at all. A merged model, where the geometry really is combined into a single file, is a different thing with a different, much narrower use case.