In January 2016, Microsoft launched a limited time promotion - the
Name Your Game Bundle - in which the buyer of a new Xbox One console would receive a code to select one of five digital games free. Each of the games was either published by Microsoft or had a marketing tie-in with Microsoft - and these were codes, not shipped discs - so this allowed Microsoft to keep costs down somewhat for this free incentive.
I think we see that again. I think all Day One pre-orders receive a code which allows the console buyer to choose one free digital game. Titles could potentially include Sea of Thieves, Crackdown 3, Middle‑earth Shadow of War, Halo Wars 2 (already released), Cuphead, and Forza Motorsport 7.
I would also expect codes related to the Fall 2017 update of the Xbox dashboard. Free stuff related to (the brought-back) avatars, new backgrounds, and other frilly customization options we'll be hearing about as we get closer to the next Windows 10 Creators Update.
The first free item says "Here - have a free
game that is optimized to use the most powerful console ever made." The second (mostly meaningless) items encourage the player to make the
device their own: more options to outfit their console character with, items for bragging rights
('day one'), and ways of making the console dashboard seem more distinctive and inviting
(a la PS4 'Themes').