What is the importance of the "sleeping pill powder" in the novel, Danny the Champion of the World?

Danny and his father use the sleeping pill powder to lace raisins to feed to the pheasants they intend to poach. The sleeping pill powder is divided up and sewn into raisins, which are then collected in sacks. The sleeping pill powder puts the pheasants to sleep after they eat the raisins, which in turn, allows Danny and his father to easily collect and poach the pheasants.