Under what circumstances do you use a breast pump?

Many mothers choose to use breast pumps now because it is more convenient and fast, and the milk sucking milk can be placed in the refrigerator and kept fresh. Even if the mother is not around, it can allow the baby to drink breast milk anytime, anywhere. Health also avoids the embarrassment of mothers' feeding in public. So, what is the breast pump?
The purpose of purchasing a breast pump is not to replace direct breastfeeding. In order to use a breast pump:

First, the baby is born early, mother does not necessarily have sufficient milk, so in addition to letting the baby suck the mother's breast, use the breast pump to suck breasts, breast milk is relatively early, and the amount is relatively more.
The second is that the mother's milk is enough, but it cannot be fed directly for some reason. If you need to go to work, you must use a breast pump to suck the breast milk during the work and place it in the refrigerator and save it as a baby's food.

Third, the mother's milk is too much, and the baby can't finish eating. You need to use a breast pump to take the milk.