In addition to what Ruz has mentioned, this is also done for the purpose of smoothing out vertex lighting. Many independent developers still cannot afford the time needed for generating normal mapped content, and so having a few props with normal mapping and many others without will results in a large inconsistency. As such, you model with the knowledge that the end result will not be receiving per-pixel lighting and you can effectively control the shading much more.
These meshes are built in such a way that I can easily go from low to high poly without losing shape, and if necessary I will not need per-pixel lighting in order to maintain a consistent and good-looking shading across the surface of the objects.
Again, take a cube and set it to entirely smooth shading -- you'll find it's not that pretty. If you chamfer, or bevel, the edges then you'll get much more consistent and smooth but useful lighting that will not stand out, for instance, on a crate.
Oddly enough, I still haven't modeled a crate, so here's a dumpster.
Click for wireframeAnd here's Krusty the Clown.