when i was living in the hood a few guys was doing it and one of them lived in a building across the street from mine so i saw them all the time and as far as i could tell they did it for the same reason some people raise and train different animals. with pigeons i don't think it's that hard to do. since they're scavengers you just have to attract a few with food in a coup let them go to fly around a little and put more food in the coup and it's a garantee that they'll be back.