If they want to monitor the populations and keep track of individuals, yes, it's necessary, Barkingdog. One adult falcon looks pretty much like another of the same gender--I'm sure the birds can tell each other apart but for us mere humans, it can be difficult.
Since peregrines are just starting to make a comeback in many areas, it's important to keep track of things like breeding success and where individuals disperse to. Banding is the easiest and least invasive way to do that.
Amazing how quickly they grow, isn't it, Cindy!
Holy catz! It's kind of sad to think that soon they'll all be fledged...