Maybe I can add to that.
Part of your question is how we got to the place we are now, where there seems to be so much use. Part of the reason was that antibiotics were such effective agents for controlling, preventing, and treating diseases that they were widely adopted in animal medicine, as they were in human medicine, especially before resistance problems started to diminish that.
The other big factor in veterinary medicine, which differs from human medicine, is that their widespread use for growth promotion was a major factor that led to the high volumes of use there. That then tipped over into disease prevention because it's thought that they do both things.
In the early years, it was demonstrated that they had about a 10% benefit to the rate of growth and efficiency of feed. That has diminished a lot—almost down to 1% now—and they were relatively cheap, so they were a cheap and effective mechanism for enhancing production. I think those are the major drivers for how we got to the place we are at today.