Take a closer look at your edges:
You can see, that you havn`t changed the shape of your box as if it was a chamfer-box. And you have nice "natural" non-perfect edges !
Ok, so what does this give to us in our daily CG-Business ? Here you can see a part of a model with no smoothing on it:
With the smoothing modifier on the object:
as you can see : hard, geometrical object with nice smooth beveld edges :-)
These edges are solid-beveld with additional edgeloops to keep the shading-gradients at the geometrical edge.
This technique offers modelling with a small count of polys and a nice beveld edges, too.
Hope this tutorial isn`t too basic and it helps understanding the technique of Poly-Smoothing. It is very usefull for technical stuff not for organic objects....
Here is a maxfile (max6) , i prepared for download. It is self explaining when you open it :-) solid_bevel.zip