Roethlisberger is an elite quarterback. He doesn't, however, play a full season regularly. As a true team guy, I'm sure he'd feel comfortable if the team found a young player to groom behind him as his backup. Right now, there is no backup.
Wilson is the perfect fit for that. He doesn't project as a starter in the NFL, at least not right now. He is, however, a playmaker. He could end up being a great backup or he could become a trade piece if he develops into a guy who needs his own team.
The Steelers would be wise to get a backup guy in the draft. They've spent far too long and far too much on quick fixes that don't really work. Wilson should be that guy in the fourth or fifth round.