Is that a rule with no purpose? I can't imagine that pine tar would increase the ball's flight off the barrel of the bat. Seems so inconsequential, unless I'm missing something.
It was useless. Just a generic rule that no foreign substance could go more than, I think, 18" from the handle. I'm not sure what they were guarding against other than maybe gobs of pine tar sticking to the ball.
People forget that the MLB commish actually overturned the ruling, based on similar reasoning that it was meant to save balls. The HR counted and they finished the game later in the year picking up after the HR was hit, with the Royal's winning. The yankees tried to appeal every base and the umpires had a signed statement from the crew of the original game that Brett had touched every base. The Yanks protested the game, but the result stood.
I actually remember that well now that you reminded me.