The man ordering coffee says, "I want coffee without cream." The waitress replies, "I'm sorry, we've run out of cream, but I can still bring you coffee without milk."Zizek makes the point that in this exchange we see the importance of the representation of absence. Even though black coffee is black coffee, whether caused by the absence of milk or the absence of cream, the specific absence makes the difference.
I think a similar logic is at work in the question of existential goodness. In other words, a person might give aid to a needy person because they follow the Christian command to love, and another person might do the same thing because they've concluded that it maximises the total amount of happiness in the world. The distinction between the two is only the unseen motivation.
In a pragmatic view, the two should be judged the same. The need was met. Are we then just quibbling about the details if we consider motivation? How about a third case?
A person gives aid to a needy person because they know that doing so they will make the needy person dependent on aid, and later can exploit this dependency. The need was met, but the motivation is vastly different.
The representation of absence can be quite important, especially in cases where something is defined by its absence, or defined by the absent motivation. Sometimes it's a necessary addendum, albeit secondary. After all, there's no point ordering coffee without cream if you don't even get the coffee at all, whether without cream or milk.