The obvious answer is: You can't profitably slaughter diseased animals (at least for human consumption, with various complex exemptions that massively reduce value)
So treating with broad-spec antibiotic prophylaxis increases your healthy-at-slaughter-date yield, especially under cost-optimised industrial feedlots.
More incrementally, (if perhaps still somewhat controversially), normal gut bacteria accounts for some fraction of the energy intake provided by animal feed. By killing them off, it frees up nutrients for the host. Other mechanisms including reduced immune load may also be involved.
Quoting from [1]:
> According to the National Office of Animal Health (NOAH, 2001), antibiotic growth promoters are used to "help growing animals digest their food more efficiently, get maximum benefit from it and allow them to develop into strong and healthy individuals". Although the mechanism underpinning their action is unclear, it is believed that the antibiotics suppress sensitive populations of bacteria in the intestines. It has been estimated that as much as 6 per cent of the net energy in the pig diet could be lost due to microbial fermentation in the intestine (Jensen, 1998). If the microbial population could be better controlled, it is possible that the lost energy could be diverted to growth.
User shabble has some ideas (in a sibling comment to this one), but reading that gives me the impression that this is a little bit of a theoretical concept. It's not. It's very widely practiced, with the explicit intent of "promoting growth".
E.g., a search like [1] calls up a large number of references to it.
