You buy the
digital download version of the full (CD) game for that.
Many folk prefer the electronic download over buying the boxed CD since that involves postal charges, and some countries Customs officers slap import taxes on the shipment. Plus its ready to install right now rather than waiting a week or more for the postman.
cheers
Andy
(Edited as I had thought you were talking about the digital download - not the Windows Safe Box edition
)