It's often a matter of scale.
If you're the author of your models and you're working in NX then you can do much of these same visualization type operations inside of NX. In fact, over the years we've evolved the graphics behavior and capabilities of NX to be more like Vis Mockup with the idea that if you already have NX that you may just wish to stay in NX and do ALL your work there, and up to a certain sizes of your models, this is perfectly feasible. However, unless your final products are not too large, in terms of the number of parts, eventually you're going to reach a point where it's impractical to try and do this with your entire final product, fully loaded and visualized, so that you can see all aspects of your product, check interferences, do 'fly thrus', etc. For example, if you're an automotive OEM or a you build aircraft or even large pieces of heavy machinery, while the sub-assemblies and major subsystems could be handled inside of NX, the entire model might be too much for it, then you might wish to consider using something like Vis Mockup to do that sort of work, NOT authoring in the sense that you're actually creating and editing geometry, but rather doing things like verifying results, looking at relationships between major subsystems, conducting design reviews with colleagues, suppliers, customers, etc, often Vis Mockuo provides a more suitable environement for these sorts of tasks. As for what sort of role is the user of Vis Mockup playing, well that often depends on where in the design, engineering, manufacturing process are you at the moment and what it is that you're attempting to learn or do.
Anyway, I hope this helps.
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:
To an Engineer, the glass is twice as big as it needs to be.