Normally, you'd have to play your way through from the beginning to the end of a campaign, but Treyarch is doing something different with Call of Duty: Black Ops 3.
Right from the get go, you can play the last level, or any level if you so desire. Jason Blundell tells Eurogamer that they won't follow the traditional way of locking campaign levels unless you go through them in order.
Comparison's to Netflix's model if giving you all the episodes at once for shows like House of Cards or Daredevil are natural in that you are allowed to consume it in any order.
What about people just uploading the ending right away? There's really nothing to prevent that as that's happened with many games in the past. Just with Black Ops 3, you can experience it yourself now as soon as you get the game.
I would guess that most people who are interested in the single player campaign will want to go from start to finish, but the choice is there for you to pick at whatever level you want to play from the onset. We'll see if this starts a trend in other games as well, but this will only work in certain types of games, of course.