Blue Jackets GM Don Waddell worked to add depth to the organization this summer by signing some more experienced players to play for the Cleveland Monsters. Dysin Mayo is one example, and he could be a recall option for the Jackets if they run into injuries.
Any GM around the NHL will tell you that you need at least 10 defensemen to get through a full season. Every team loses players due to injury, especially on the back-end. Defensemen tend to take a hit every time they touch the puck, after all.
It's important to have depth within the system to cover for those injuries.