Peptides occur naturally in the body and communicate with the cells to perform in a certain way.
The anterior pituitary secretes prolactin, which through complicated interactions with other hormones that I don't understand, acts on the adrenal cortex to regulate the secretion of glucocorticoids, and growth hormone, which acts on bone, muscle, and the liver.
So basically they trigger the body to make GH naturally. Which is cheaper, and reduces the side-effects/risks of introducing Growth Hormone into the body exogenously.