Barn cats are good for keeping down the rodent population. That is their general or intended purpose. Their presence is a rodent deterrent. Barn cats replicate the original domestic cats going back almost 10,000 years.
However, they need some management. Perhaps the first issue is that some people will say that semi-feral cats are not very good at rat catching. I tend to agree with that. However, it depends upon the individual cat. There have been some prolific rat catching cats. But as mentioned their presence is a deterrent. They also catch bugs, snakes and mice.
A very well known celebrated working cat, Palmerston, living at the Foreign Office in the UK has been given a grade A performance rating after catching 30 mice and one pigeon in the past 12 months. In the previous year he caught 26 mice and one rat. Palmerston is a domestic cat and is doing his job in keeping rodents at bay in a very large office in the centre of London.
Barn cats will need feeding and when you feed cats outside of the home you are liable to attract wildlife such as raccoons, possums and skunks. I suppose the food might attract rats too. Along the lines of a well-run TNR program it is probably wise to put food down for a certain period of time and in a certain quantity and then remove it daily. The objective is not to leave it down all the time but to manage the provision of food.
Also, it goes without saying, that the cats need to be spayed and neutered unless you want a fairly rapidly increasing population of barn cats living on your farm. You will also need to take care of their health. Such routine treatments as deworming will be required, ideally, together with standard checkups and vaccinations. You can’t treat them as if they are entirely self-sufficient.
Another reason why barn cats are good is because sometimes suburban feral cats are relocated to barn environments. On this occasion it is good for the cats. Of course, feral cats should not exist in an ideal world because they are the product of irresponsible cat ownership. As that problem will always be present in varying degrees then people have a duty to act humanely towards feral cats.
Another point worth making is that barn cats can provide companionship to farm livestock. There are many photographs on the Internet of barn cats befriending horses, cows and any other farm animals to whom the cats have been socialized. Horses and barn cats go together very nicely. So if you keep horses barn cats are good for providing them with a potential companion.
The general consensus is that barn cats are a good thing.