There are three uses of antibiotics in veterinary settings:

1. Treating actually sick animals 2. Low-dose treatments to promote growth 3. Prophylactic treatment to prevent initial infection

Both 2 and 3 are widespread, and a problem.

They keep the animals in unhealthy, unnatural conditions where they will get sick more often. The antibiotics are there to combat that and the side effects of eating corn (in the case of cows) make them very unhealthy and prone to getting sick as well. The whole thing is a mess.