Its basically to let everyone know whose turf it is. They crow to let the girls know they are there, they crow when they see someone or something new come in their turf, they crow to send a message ect. Roosters are vocal and crowing is their way of sending their point across to whomever and whatever is around.
I raised some peeps in my greenhouse before the new coop was built. Yup. When the moon was full and round, and the night was clear and crisp all 8 roosters would crow all night long............................................
Buckles seems to have a built in alarm clock. I let them out to free range for an hour each night and stay with them because there are kazillions of hawks around. When it is about time for me to let them out of their coop he starts to crow. I am at the point still inside the house so I guess he is trying to remind me?
My largest Roo would crow when he heard me coming. None of the others have yet. We just had to cull the Roos the hens didn't prefer. I already miss his calls of good morning. :-( I'm hoping one of the remaining will learn to crow.
He's trying to make his territory known. Notice that the crows do vary. Sometimes the crow is to warn the hens of danger (that's when you see the hens running in a panic). I have a stupid bantam that crows in the middle of the night.