Microsoft's much-aplauded backflip regarding Xbox One online requirements means that some games won't work quite right straight out of the box. Take Forza 5 for example: The game relies on a (once-off) download, as the fellas at Turn 10 have chosen not to put the entire title on the disc.
Dan Greenawalt explains:
When you first boot up the game, we’re going to ask you to log in, and when you log in you’re going to get the Drivatars and you’re also going to get a whole bunch of content: Tracks and cars. Our production schedule is such that we are putting them in as late as possible and that means making them free as downloadable content on Day One.
We're guessing that in this age of digital downloads, Forza 5 won't be the only game to use a model like this. Greenawalt points out that they're making "the biggest game possible", and packing it all onto the disc simply won't work.
The initial download is required - you'll need it in order to play the game. After that, you can throw away your network cable if you really feel that way inclined, the game is perfectly happy to run in a completely offline state.
...but as Greenawalt tells IGN, you'll probably want to keep the internet handy. The in-game "drivatars" will perform better the more user data they are fed - and of course, those updates can only happen online.