The reason is injuries. I think we all roster, or at a minimum the majority of the league carries injured players over from year to year, I don't think we should have to penalize ourselves to carry these injured player for an extra day/2/3 before they get put on the DL by the big league club, when we all clearly know that's where they're going.
So I'm not proposing a permant expansion to 28, but just for say the first week of the season. Reason being, to get those players that are still injured to the DL without having to drop a different player, valuable or not.