There is just no point to include digital codes to a box version if you have to open the box to get them.
Maybe there will be one seal around the DVD and another around the codes and sleeve, but if the codes are accessible outside the packaging, at some point during the distribution, storage or stocking process, some portion of the codes are going to go 'missing'.
whether you buy phyiscal or digital you still need the code to activate the game
There is no activation required, though you do need the Steam code use the Steamworks features. The GOG code is a DRM free digital backup (and possibly a way for people who do not have Steam to get additional languages or an eventual Linux version, if GOG adds Linux support by the time it is released).