Note that with the exception of let-down, all other causes are not routine, acceptable reasons. A mother with less storage capacity for milk in her breasts may also leak more as her body attempts to produce the quantity of milk demanded by her growing baby. Perhaps leaking is helpful for the new baby who is still learning how to nurse efficiently. The release of the milk provides multi-sensory input to remind him why he is there. He may see the milk drip, it may dribble in his mouth, he may smell it, or feel the wetness.
Sensory input is especially helpful for the sleepy or reluctant nurser.
In a natural setting, baby would be with his mother, so he could signal that it is time to nurse. Of course, the reality today is that many babies are not with their mother every time they want to nurse, so women are likely to leak. But it could be argued that this still is not a situation that is supposed to happen. There are some situations that cause mothers to make, and consequently leak, copious amounts of milk. Certain hormonal imbalances, pituitary tumors, even medications can put milk production in high gear.