As soon as we knew Castellanos wasn't making the team it seemed incredibly obvious that it was going to be Dobnak to fill that roll. They want someone to potentially eat innings, especially early in the season. Say what you will about Dobnak, but he's pretty effective in that role. That is how he was used this spring and he was effective doing it.
As was stated above, due to his $3M guaranteed salary, if he does just a so-so job in that role, it's easy to DFA him, and have him get through waivers and back to AAA. If he does well in that role, then great, we've got a guy who can eat innings and keep our higher leverage relievers fresher than they have been the last couple years.