As JoeyA said, so the players can prepare. At one pocket, the players often consider the positions of every ball on the table when planning the current shot. A ball alone on the spot can be a huge difference from no ball on the spot or two balls on the spot. If both players agree, the ball can be spotted early.
This is very true,there were more than a few times,that i forgot i had to spot a ball up after my shot,and it can and will change the layout of the game,BOB your idea is the best and fairest way and the correct time to spot the forgotten ball.