While nutrient-shuttling is the main function of insulin, it's not the rate-limiting factor in a person using a moderate dose of steroids. Nutrients can make their way to cells just fine. It's not like in a GH user where they literally need the insulin to absorb nutrients.
Instead, I think the anti-catabolic effect of insulin is the important thing. Taken pre-workout, insulin will sway the balance of anabolism:catabolism towards the side of anabolism...essentially, less muscle gets broken down during the workout. So that's why the insulin is important. Now we can't take insulin by itself obviously, but taking too many carbs will just make us fat. So we want to take just enough carbs to cover the effects of the insulin.
Theoretically, you get the muscle-sparing effect from a big influx of insulin without having consumed as many carbs. Theoretically, this would be advantageous when trying to retain muscle while keeping calories/carbs low.
That's the logic behind it....does it pan out in real life? Who knows.
