3 Answers

the more roosters you have, the more they will crow.They crow because the other one does. Make sure there is nothing stressing or scaring them to cause them to raise an alarm. during the day or at night. you might want to check well for mites or lice which could be irritating them at night.(best time to check is at night using a flashlight) if they are in an open type shed coop, even car lights going by will make them crow.

Why

to mark their territory from other roosters they might hear in the distance or to each other.
to raise an alarm or distress.
to call the hens.
because they are roosters and that's what they do.

To show the girls how smart and loud he is.

He's advertising that He's proud to still be  horny in the morning having been at it alnight. Prowess