They crow triumphantly when they think they chase something away, a person, a predator or another roo; they crow to communicate with other roos, to call the hens, because they're happy to be alive and it's a nice day, a million other reasons
Roosters crow in the morning because they can sense the ultra-violet rays of the sun well before our eyes can even see a little bit of light......at least that was what I was told!
I have another question to add: At what age will my hen crow if she turns out to be a rooster?