To be honest and in my experience only, the only sponge Ive removed is the black carbon sponge, purely because they dont last long as in usefulness, the only reason to run carbon is remove medication if/when you need to medicate the tank, so take out the black sponge and keep it for use if you need to remove meds (will save you buying one). I havent removed the blue sponges to use the box and I dont really know why you would, but each to their own, the green nitrAte sponge as said doesnt really do anything, its suppose to remove nitrAtes from your tank but if it does, I havent noticed any difference and the only reason to remove it would be if your planning a heavily planted tank as the plants will use up the nitrAtes.
As I say its just my opinion/experience with the juwel internals (ive had two, still got one and will have one in new tank next week also).