who else is going to kill those billions of old and ill weak and dying birds? In wilderness this is done by a plethora of predators. In developed areas pretty much only cats are left to perform that duty. Without cats it would probably be the rats walking around eating the finally fallen dead birds lying around. Your choice.

And don't forget another aspect - rats eat bird eggs imparting massive damage at that stage, and the only predator available that can effectively control rats in the areas inside and surrounding human developments is cats.