the idea of a 1/3 superfree with your meal is to stop having a plate filled with just carbs and proteins, which although are still free on extra easy, they are the foods that are higher in calories and if you do overindulge in them with not enough superfree, then it is likely you will be consuming well over what you body physically needs and may get no weight loss or at the worst even a gain.
Slimming World of course is not a calorie counting diet, but it still uses the same principle, just in a more clever way without having to measure or work out points/calories etc.
I always say I dont agree with eating the 1/3 superfree AFTER your meal, the reason for this is, you could eat a plate piled high with just carbs and proteins, be completely stuffed and then go on and eat a bowl of fruit because you are thinking "I must have my 1/3 superfree" - so with that in mind I think your 1/3 superfree should come before or with you meal always. It is far better to be satisfied and leave some on your plate, than to clear all your plate, feel completely full to almost bursting point and then force yourself to eat a bowl of fruit.