The basic premise is that when you engage in aerobic activity for a long time (racing 10+ miles at a decent speed, long distance cycling, etc.) your body exhausts the energy/glycogen stored in your muscles.  The Gu is basically pure sugar and is designed for your body to really quickly replenish some of the spent energy.  You really only need them them on super long training runs or longer races.  Of course that doesn't stop fatties walking a 5k from consuming them in mass quantities.