I'm saying it has online functionality built into the game and many people will be playing it online with other people. Just like they do on the PC. Online as a feature is going to be a staple in the coming years.
The ability to play offline will be utilized by a small subset of the player base.
Initially the Console version was also supposed to be online-only, but Blizzard's research showed a huge part of the Console population didn't have an Internet connection
http://www.diablofans.com/news/1873-exclusive-fansite-console-info-from-blizzard-hq/
And..
It supports 4 player local co-op (and online co-op) of course.