Stapleton required a belonging of Henry's in order to train the hound to attack and kill him. Stapleton was a distant relative, a Baskerville with his eye on Henry's property. He had similarly killed the previous Baskerville who had been living there. So he stole one of Henry's boots to use to train the hound. But it didn't quite work because the boots were new and the hound didn't pick up Henry's scent well. Ironically, Stapleton was still attacked by the hound.