STL files are typically created from scanned data, laser, faro arm, ect, to collect point cloud data. The point cloud is then converted to a parametric mesh, there are various software packages that can convert the mesh to usable surfaces or solids. Surfaces have defined edges, Solids have faces, features and edges,... Ok so what does that do for me? With defined edges, faces and features and CAM software I can generate toolpath data to program any number of CNC machines.
Can I toolpath an STL? Yes, in some cases however not nearly as efficient as working with solids, surfaces, lines, arcs and splines.
Just for clarification "dumb solids" are models with no history, if you save a model out of SWorks as a step, x_t parasolid or iguess the sldprt history is gone.