The reason is to have a complete set of scenarios. Rule 3-3-2-f says that if there are d) and e) reasons for the clock to have stopped, the clock will start on the snap. In your first example the clock will start on the snap, and in the second example it will start when it would have started without the DB DOF foul.
The rule used to just say (something like) "if the game clock is stopped and it will not start on the snap, it will start on the referee's signal." I don't know the reasoning to have a complete list instead of the old language.