According to the HSUS, there are many ways you can help these cats that can really make a difference. See full excerpt
Helpful summary of FAQs about Outdoor Cats
Community cats who have been TNVR are a great addition to a back yard
Community cats can be considered as working cats. Why? Because they help get rid of rodents, mice and rats, which are in your yard and will get in your garbage, garage, sheds and sometimes your house. In Chicago which has been plagued by an explosion of the rat population there is a 3 months waiting list for community cats.
Having a colony of community cats in your back yard is a win-win for you and for them. You give them food and water and provide them with a shelter and in exchange they will be the guardians of your yard. And no not all the neighborhood cats will end up on your property. Cats are very territorial and will keep away other cats from staying on their turf.