The idea is that the aircraft may be filling up with water if it has ditched. You inflate your life jacket in the aircraft and you will float to the top of the water inside the the cabin. If the water has come to the top of the door, you will find it impossible to get out because your life jacket will prevent you from ducking below the surface in order to get out the doorway. Even if there is no water in the aircraft, as you go out the door, a wave may splash against the doorway. With an inflated life jacket on, you will find it very difficult to get out of the door in those circumstances. And finally, moving about the cabin with a big balloon on you is going to severely hinder your mobility and hence the speed with which you and your fellow passengers exit the aircraft.
So you are best to only inflate your jacket when you are certain that you are not going to end up back inside the doorway.