why you use sub-div surfaces in conjunction with polygonal modeling

I have been learning how switching your model between polys and sub-divs can really let you model in a whole new way. the best way i can think to describe it is that modeling with polys is like carving out of wood, which is the best way to build up a primary form to work with. Then sub divs are like changing your model from wood to clay, alowing you to sculpt curves easily without creating creases  or unwanted hard lines in your work. by using a combonation of the 2 i think you can very quickly get the results in your model that you are after. Here is my video explanation of the two, displayed by first a poly model, then the same model after around 10 mins of sub-div editing: