Antibiotic Growth Promoters (AGPs) were first licenced for use in farm animals' feeds (US) in the 1950s.
Since then :
“Supplementing animal feed with antimicrobial agents to enhance growth has been common practice for more than 30 years and is estimated to constitute more than half the total antimicrobial use worldwide.”
AGPs increase feed utilization (in other words, the animals need less food) and also cause the animals to gain weight (typically up to 10%).
The exact mechanisms by which this happens are still not determined. One current hypothesis is that the antibiotics depress the growth of adverse bacteria in the animals' gut.
“[…] the mechanism of action by which antibiotics promote growth has not been fully determined, so specific effects on animal and bacterial populations to be replaced by alternatives are not well defined. Moreover, the effectiveness and cost-effectiveness of antibiotic growth promoters are not well understood.”
Source: Alternatives to Antibiotics in Animal Agriculture, PEW 2017
Despite this lack of understanding, and (proven) concerns about their role in the spread of antibiotic resistance, AGPs are still in very widespread use in animal farming worldwide :
“Whatever the mechanism of action, the result of the use of growth promoters is an improvement in daily growth rates between 1 and 10 per cent resulting in meat of a better quality, with less fat and increased protein content.”
Note: AGPs are now illegal in many countries (with only partial ban in the USA).