At the time of death in an animal dying of anthrax, the blood and body fluids are loaded with anthrax bacteria: there are more bacteria than red blood corpuscles in the blood. On exposure to the oxygen of the air, these start to form spores - the hardy forms of the anthrax agent which can survive for decades in the soil where blood and body fluids from such an animal have spilled. Vultures, in demolishing the dead animal before much of this sporulation can occur, contribute to reducing the environmental contamination left by the carcase. In theory, the faster the carcase is consumed, the less the chance of sporulation and residual contamination. And, at least to a point, more vultures = faster consumption. Faster consumption aldo reduces the chance of spread of the disease by flies.