As mentioned, I keep them because it dramatically enhances the resale value. I don't keep them for other commodity items that aren't likely candidates for resale.

People want the Nikon and Canon packaging as it helps to provide provenance, especially in regard to proving the object in question isn't a grey market import.

Did keep the box for my big TV in the basement, but that was largely due to the fact that I know I'm moving in the relatively near future and it would better protect it during the move. ;)