Graves W is possibly the best CC in the entire game

"But , you can just walk out of it, it doesnt do much."

Consider a knockup, usually agreed upon as the best CC because it cant be d or d. When this happens, you are able to prepare your flash, your escape, your skillshots, your whatever as soon as the CC wears off.

What happens when hits you with his smoke screen? You cant see, so unless the is a retard and stands in his own smoke, you cant attack. You cant use targeted dashes. You can use dashes that dont require a target to quickly get out of it, but to where? Where is Graves? He can easily reposition himself and now youve put probably your only gap closer (or even !?) on cd when you dont even know where he is. If you guess wrong, youre already screwed.

If you simply try to walk out of it (while slowed), it will take an average of at least 2 seconds to walk out of it and get a reading on your surroundings to figure out your next move. When youre stunned or knocked up, you can plan your next move. Graves smoke doesnt allow that, so you and anyone else hit by it are essentially nullifed for more than 2 seconds.

Remember that "hidden power" Riot says they dont like? Here it is.