It's becoming an increasing trend for AAA titles to have less focus on the singleplayer aspects and more focus on the multiplayer. This is especially the case for many first person shooters over the last few years.
The first game to see a multiplayer only experience was Titanfall and since then we've now got Battlefront and it's now been revealed that Rainbow Six: Siege will also have no singleplayer campaign. This means that, besides the co-op mode Terrorist Hunt, everything in Rainbow Six: Siege will be on a player-versus-player basis, and almost everything will be done online either with or against other players.
There are a couple of exceptions to the above information. Rainbow Six: Siege will have a few training missions as well as a versus mode against AI for offline play, but these are the only offline options for players.
This information was revealed by Siege's art director Scott Mitchell, who said that players will, "Go through training, where you get to experience different operators and their devices. You can play against enemy AI in co-op through all the maps. You can customize matches, so that's what we're offering on the single-player side of things."
At this point it's not uncommon to see multiplayer only titles so the latest news about Rainbow Six: Siege doesn't come as much of a shock. A lot of players are unsure of the costs of the game though, which will set players back $60 on all platforms. Would you be willing to pay $60 for Rainbow Six or would you rather spend that on another game, such as Fallout 4, Black Ops 3 or Halo 5, which will have a story campaign?