The problem with insulin is that while it can spur on anabolic activity, if you are prone to it then it will also happily drive your body to store fat.
This is especially true in individuals who are either insulin resistant or who have eaten more carbs than their muscles are able to readily absorb.
Generally, the dogma is that you want to ramp up insulin after a workout, to stop catabolism and spur anabolism.
For many, especially those trying to lose fat, insulin stops the release of fat from storage -- making it very very hard to lean out. Controlling the amount of insulin released during the day by keeping the GI/II low can help allow fat loss.
I'm probably butchering the concept, but this is the gist of it.