Quick help!

I have the game on disk, but want to activate it via Steam. But now Steam is downloading the game, shouldn't it install from the disk as the data is already on there? I thought Steam would just be a means of activating it and that I wouldn't have to download the game through Steam. Just wondering if this is the case or if this is a bizarre occurence? The game is a legit copy btw, if you activate it through Steam does the game have to download again through Steam, even though you have the disk with the data on?