Why do some bodybuilders use insulin as a supplement?
First, I'd hardly call synthetic insulin a "supplement." it is really a serious drug, not meant to be used by bodybuilders but those inflicted with insulin-dependent diabetes. Insulin is only available by prescription, but that has never stood in the way of muscle hungry bodybuilders that want a specific drug.
The reason that bodybuilders are interested in insulin lies in it's ability to cause the storage of greater amounts of carbohydrates and amino acids inside muscle cells. Insulin is both anabolic and anti-catabolic in nature, and can produce dramatic and rapid increases in the size and fullness of the muscles when used "correctly."
Insulin also has a synergistic effect with other bodybuilding drugs like growth hormone and steroids, each helping the other drug to work more efficiently.
I should also mention that because insulin is not detected by standard urinalysis testing, it is also a favorite among "natural" bodybuilders as well. Insulin, along with HGH, which is also undetectable, are used by many that compete in drug tested events. This is cheating at its worst of course.
Bottom line, synthetic insulin is a powerful and dangerous drug. When used "correctly" it can certainly enhance hypertrophy to astounding levels. Used incorrectly on the other hand, it can make you fat, or far worse, kill you.