The 3mf format has nothing to do with Grasshopper. As you noted, it is one of many formats Rhino can use to export geometry. The exported file is then used as input to some other piece of software for either further development or actual production.
I don’t know what the precise nature of 3mf files is, but I do know they can be used by software that generates toolpaths for production machines such as 3D printers, milling machines, laser cutters,etc. The STL format is also commonly used for this same purpose, but requires a much larger file to represent the same geometry.
Rhino has the ability to output any of it’s numerous Export formats regardless of how the geometry is structured in Rhino; it can be either meshes or NURBS or a combination of both. I’ve created 3D printed parts from STL files comprised of both NURBS and mesh geometry. For some reason it seems that Rhino can export geometry faster if NURBS geometry is converted to mesh first, but I really have no idea if this is always true.