Facetted geometry is a type of import/output format where surfaces are broken up into polygons at various levels of accuracy. Historically they were used for world file (.wrl) and very popular in the VRML language. There is a lot you can do with them in environments that can make use of the data for 3D graphical presentation purposes. They are extremely lightweight on the CPU and graphics engine making them easy to display huge data files. There is also some very powerful features within the VRML language such as smoothing, color, and texture mapping. It also has a level-of-detail (LOD) algorithm as part of the standard to let the software determine how detailed it is to present the information. The graphics engine in Creo works on a VRML standard for shading.
In Creo, facet files are nearly worthless as imports. You simply cannot do anything with them other than pick up a few vertices or some other referencing. In general, avoid them for any useful levels of work.