From what I understand, it isn't usually used as a growth enhancer (I may be wrong) but used because of the way livestock is maintained. When you have cows being kept standing in one place to maintain lean meat, they develop infections more so than being able to walk somewhere other than their own feces. So to keep production up and infections down, the livestock gets pumped full of antibiotics.
Huge amounts of ionophores [1] are used as feed additives. That class of drugs is not used therapeutically in humans. I think other antibiotics are also used quite extensively as feed additives, but I'm less clear on them. If you search on 'vancomycin feed additive', you see discussion about replacing it with other drugs.
There are health benefits to the animals (the drugs prevent 'bloat'), but they also increase the weight gain per pound of feed, which the farmer (rancher? lot manager?) is going to take into account.
