I can't say for sure on the why. But I would suspect it was for several reasons. To keep his wife feeling like she was a part of his game; to keep her interested in the shot by shot play; to challenge himself and keep his own mind sharp about just what he was doing.
He wouldn't do that all the time, but after his death, his wife revealed that was a little 'game' they played together. She said it didn't matter if the ball she picked ended up on the rail at the wrong end of the table, he would still get it positioned down by the rack!