What is the justification for use of antibiotics in food-producing animals (such as cattle, hogs, and poultry) during their life? are they necessary?

The justification is that people want to mass produce so they use them to thicken them up so they produce more. They aren't necessary ... nature can work it's own way but as the worlds population grows, it's essential for farmers to make a living so naturally produced food can be offered to all for a reasonable price.