Many modders still prefer this version because it supports a vast library of existing mods without the complexities introduced by the newer "Creations" menu update (1.6.1130+). If your game has auto-updated past this version and you wish to return to it, you can use the Steam Console Downgrade Method to revert your files.
This is the part that technical players care about most. Update 1.6.640 wasn't just a content patch; it was an engine port. skyrim update 1.6.640