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.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4638249/

[–] Sheldybear 7 points 6 months ago

Don't worry, it gets worse! Certain farmed animals are particularly susceptible to infections like shrimp and oysters. These animals are kept in open water pens and antibiotics are routinely DUMPED INTO THE OCEAN to protect the stock, naturally contaminating the ocean at large and giving bacteria in the wild just enough exposure to antibiotics to develop resistance.

If you want to support responsible antibiotic use, avoid all farmed shellfish, don't buy any meats from India or China, and only buy free range chicken; these are the biggest global offenders. If you're European, avoid meats from Greece or italy.