While carbs definitely DO play an important role in your diet for various reasons and for various goals (especially building muscle, increasing strength or improving performance), they are the least important of the macronutrients. Carbs Are Extremely Useful, but not essential. While protein and fat (specifically the omega-3 and omega-6 fatty acids) are essential to your body, carbs on the other hand are not.
Despite the fact that the glycemic index was originally created for diabetics as a way of figuring out which foods would be best for them, it quickly became used by bodybuilders, athletes, and regular people who just want to look good and be healthy.
Why? Because the glycemic index allows us to maintain steady blood glucose levels throughout the day by choosing the right types of high carb foods at the right time.
This of course is useful for many reasons, the most important of which are controlling hunger, maintaining energy levels, improving the way our bodies look/perform and preventing a variety of health/medical issues associated with frequent and sustained spikes in blood sugar levels (like type 2 diabetes and heart disease).
A carbohydrate with a high glycemic index (high GI) breaks down quickly during digestion and therefore releases glucose into the bloodstream rapidly, while a carbohydrate with a low glycemic index (low GI) breaks down more slowly therefore causing a much slower and more gradual release of glucose into the bloodstream.
While complex/low glycemic foods should comprise the majority of your carb intake, your post workout meal is the one time of the day when simple/high glycemic foods may actually be the better choice.