User blog comment:Lancer1289/BioWare gets on the Multiplayer bandwagon/@comment-809585-20111011184555/@comment-3268946-20111012013507

There is a separate story structure for the co-op, basically you are also fighting the Reapers in a different place so that the overall fight in the main campaign is easier. Like I said it is in no way required, but who doesn't want to play as a Krogan and destroy Cannibals with your friends?