Although it's becoming less and less relevant as advances in bandwidth exceed the pace at which games swell in size*, it's all about the preload. People want to be sure it's all downloaded and installed the moment it unlocks.
Preload is available a week before the game is officially available, and folk can't wait that additional 90 minutes* on launch day.
*Current UK average bandwidth would download 150GB in about 90 minutes; in 2013, downloading all 65GB of GTA5 at average UK bandwidth would have taken around ten hours.
