In co-op, playing as Carver will give you new experiences that Isaac wouldn't see. As Isaac and Carver approach an elevator door, Carver sees a toy soldier there, which is unique to co-op, because Isaac (and the user playing as Isaac) can't see it. Later, Carver hears his dead wife in an audio log, while Isaac hears something completely different.
Finally, Carver is pressed with a serious bout of dementia and has a major freakout, taking the player into his own mind, while Isaac is left to defend him from attacking Necromorphs.
It's all just a taste of what's to come from Dead Space 3 and it's OPTIONAL co-op mode!