It actually is used as a growth promoter, chickens grow quicker when given antibiotics.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC153145/

“Shortly after the introduction of the therapeutic use of antibiotics, the growth-promoting effect of these products in chickens was discovered by feeding fermentation offal from the chlortetracycline production of Streptomyces aureofaciens (122). Several antibiotics have been in use as growth promoters of farm animals ever since. The introduction of these agents coincided with intensive animal rearing. These products improved feed conversion and animal growth and reduced morbidity and mortality due to clinical and subclinical diseases. The average growth improvement was estimated to be between 4 and 8%, and feed utilization was improved by 2 to 5% (90).”