Insulin is very anabolic on it's own, with a steroid stack, and especially in a GH and steroid cycle. Basically, it fast tracks nutrients to the right places to cause muscle growth and recovery.
Unfortunately, it will do the same with fats consumed during the active period - to the body's fat stores. This is why the faster types of insulin are easier to use. Much shorter time required to carefully monitor the intake of carbs, proteins, and fats.