Historically, siege warfare was rife with attrition and starvation. The besieged had to use up their stores of water and food (unless they had some access, such as a stream/well or harbor for imports that wasn't blockaded). The besiegers were usually in enemy territory with supply lines vulnerable to being cut. Read about Caesar's siege of Alesia and you'll see what sort of logistical difficulties the Romans ran into in Gaul.
To me, the system of supply is perfectly rational. It encourages me to keep invasions under one year in length, as well as to assault fortresses/cities after only a few turns, before we get to rationing or belts tightened. Otherwise, I'd just be happy to sit there and let attrition wear away at the enemy's numbers. Once or twice, a sudden sally has broken my besieging army that was basically on half-rations and had a -2 morale hit.
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