If you need the binoculars for watching wild animals, in particular birds, the speed of focus is important. While following a bird in flight, you need to change the focus all the time if the bird gets closer or further, and if your binoculars do not allow for quick adjustment, you will struggle all the way.
Obviously, this problem does not exist in the case of “auto focus” binoculars: They stay in focus all the time, from infinity to about 10 meters away. The bird can fly as fast as he can, straight at you, and it will stay in focus. Very convenient.