Both of the two. The two main reasons are that it incidentally boost growth and there are lots of circulating diseases due to heavy overcrowding conditions. Note that the use is not on those who are sick, but to everyone even if they show no symptoms
Antibiotics are administered to animals in feed to marginally improve growth rates and to prevent infections, a practice projected to increase dramatically worldwide over the next 15 years.There is growing evidence that antibiotic resistance in humans is promoted by the widespread use of nontherapeutic antibiotics in animals.