Advantages: Cotton is typically a very very big environmentally burdensome crop to grow, because in order to get decent yields you have to apply pesticides something like 8-10 times or so in a single season. This takes a tractor, which takes fuel, and the farmer's time and money. It poisons the groundwater and is a health hazard to humans and other organisms.
Bt cotton successfully repels the boll weevil larvae, one of the principal predators of cotton, and all other larval insects trying to take a bite out of the cotton plant, eliminating the necessity for pesticide applications.