The issue is, back when scholar/whm etc. had many dots, you started fighting a boss in a raid or openworld scenario (eureka, BA etc.) and the dots didnt apply; it would show something like "Thunder III - No effect" as if the mob would be resisting it, simply because the maximum number of dots has been reached on that mob. Thus your damage didn't even contribute to killing that mob. Then they started removing dots over dots. Now it is basically helping them maintain a healthy level of debuff/dots so they can display enough other stuff on the screen.