Roosters crow because they can, also to say" this is my territory, these are my girls and get the heck out of here"......I enjoy the sound of a rooster but at times I wish they would keep it down!
I have one Silky roo that crows when someone pulls in the driveway. Very helpful. We call him the "dog". Our actual dog barks at airplanes, birds flying overhead, the farmers 1/4 mile away, not so helpful.