What is the role of insulin in the body?

Insulin is a hormone released by the pancreas in response to high glucose levels in the blood. It primarily acts to reduce blood glucose levels. It does this by signalling muscle and fat cells to take in glucose from the blood. This glucose is then used for energy. It also signals the liver to take in glucose and convert it into glycogen, allowing it to be stored in the body.
So insulin plays an important role in the regulation of blood glucose, and therefore in maintaining homeostasis.