Actually the best affiliates are ones with many offers or specific niches. Say I have a site about my dog kennel, if I sign up to be an affiliate for a couple of pet stores or sites I can easily drive sales towards those affiliates. Of if you have an affiliate that sells everything from pet food to wood floors its more likely someone will find something they may purchase. Its not as easy as a generic ad program but the earning from an actual sale is higher compared to one click through from say Adsense.