The Gemara on Amud Aleph explains the practice of offering water and food to the appointee who walks the scapegoat to the cliff through the wilderness.

At each and every booth people there say to him: Here is food; here is water, if you need it. It was taught: No man who escorted the goat ever needed this food and water. However, they would offer it to him anyway, because one who has bread in his basket is not similar to one who does not have bread in his basket. 

Rashi explains :

One who has the potential to eat feels less hunger. 

Maharsha quotes the Yerushalmi, which explains that a person tends to desire that which he does not have. 

Arvei Nachal ( ערבי נחל וישלח ב ) offers a novel application of this principle: There is a famous teaching, (Yoma 86b) “When a person commits a transgression and repeats it, it is permitted to him” כֵּיוָן שֶׁעָבַר אָדָם עֲבֵירָה וְשָׁנָה בָּהּ... נַעֲשֵׂית לוֹ כְּהֶיתֵּר.  While ordinarily, we might look at this principle in a pessimistic manner, Arvei Nachal explains that in truth, the compulsion to sin and our desires are overwhelming.  If a person tastes the sin, his desire should grow even greater.  However, as a kindness and a defense mechanism, after repeated sin, Hashem makes the object of our desire to seem as if it is permitted. Once it becomes permitted to us, our desire grows weaker, since as we have seen, אינו דומה מי שיש לו פת בסלו למי שאין לו פת בסלו that which is readily accessible to us, we desire less. So do not despair if you are mired in sin, do not think you are a basket case, because the basket will save you.