Diane, LJA thanks!
I dont know why milk is good for cleaning leaves, but remember the legend of cleopatra or someone bathing in milk? I think its because milk has some chemicals within it that nourishes the skin, in our case the leaves.
My other educated guess is that milk is a basic substances and should be a gentle cleanser.
I read that the practice of using milk on orchid leaves is to just simply dip a soft cloth or paper towel into milk to dampen it and use this damp cloth/paper towel to wipe leaves.