From:  jbshorty
You need a mesh for pretty much everything in 3D- from displaying Nurbs objects, for rendering, and for all stages of manufacturing and analysis. What you are seeing in the Moi viewport is not the actual Nurbs surface but a meshed representation of it. The true Nurbs object is infinitely smooth (in theory). In real-life this would be impractical to represent. So a mesh is required to define it in managable terms...