Why do you need the endocrine system?

Simple answer would be to survive. Hormones such as insulin, are necessary for human to function correctly because insulin lowers the blood sugar level in your body. Without beta cells, the cells in pancreas that make insulin, you would not be able to lower your blood sugar level and you will have type 1 Diabetes. There are many other important hormones such as oxytocin which constricts the uterus and causes the baby to come out and make ejection of milk from the breast during nursing to be easier.