Crows are an excellent early warning system. This time of year they're even more vigilant because their babies are out of the nest and have joined the flock. Hawks are not the only raptor they'll harass and run off; I've seen them mob owls that are minding their own business and trying to sleep. They'll also alert you to any ground predators and I've seen them chase away cats and snakes. Conceivably one could snatch a chick but I think hens and roos would put a stop to that. Keep them around and they'll alert you (and your chickens) to potential dangers.