I've always thought a rooster crows to tell his hens it's a brand new day. The cock-a-doodle-doo is man's way of imitating the sound the rooster makes. Here's more on the subject:

Animal Onomatopoeia. The sounds that animals make are complicated even in English, but it’s important to be aware that — contrary to what one might expect — in different parts of the world, the words used for animal sounds are quite different. Sheep do not universally go baa, nor do ducks quack everywhere in the world. Here are some common English renderings:
rooster cock-a-doodle-doo/crow
http://www.wisegeek.com/what-is-onomatopoeia.htm

Why do roosters crow?
Many varieties of bird sing at sunrise and it is the males of the species who sing the loudest and most frequently. No one knows all of the reasons why birds sing, but some of the activity can be explained.

Birds sing to attract mates and can accompany the song with specific physical activities. Song is used to mark a bird's territory from intruders and to warn other members of the flock of intruders. It is also used to reestablish communications with the flock after night has passed. Some species do not sing well after they first awake, but improve after a few hours practice. Studies have shown the deaf birds also sing, but their songs are distorted.

A rooster is a male bird and crowing is his way of singing.

http://www.answerbag.com/q_view/42323

Cock-a-doodle-doo. Why do roosters crow? Remember that chickens are originally from the jungle. Their wild relatives have lived in tropical forests for tens of thousands of years. Perched in the trees, and sensitive to infrared light, roosters see morning light at least forty-five minutes before we do.
Through their crowing, every rooster can recognize the crow of at least thirty other roosters, probably more. As the protectors of the flock, roosters are always on the lookout.
A shrill cry. If a rooster spots danger, he sends up a shrill cry. The other roosters echo the cry. Thereupon, the whole flock will often start up a loud, incessant, drumbeating chorus with all members facing the direction of the first alarm, or scattering for cover in the opposite direction.
All clear? All clear! When it looks safe again, an "all clear?" query goes out from the rooster, first one, followed by the others, in their various new places. Eventually, the "all clear' crow is sent up by the bird who first raised the alarm, and a series of locator crows confirms where every other rooster and his sub-flock are at this point.
http://www.upc-online.org/stories/chicken_talk.html


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Note added at 29 mins (2008-03-04 02:24:10 GMT)
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And here's a beautiful story about a rooster.

The Nun’s Priest’s Tale, Geoffrey Chaucer
A very poor widow lives in a small cottage with her two daughters. Her main possession is a noble cock called Chaunticleer. This rooster is beautiful, and nowhere in the land is there a cock who can match him in crowing. He is the master, so he thinks, of seven lovely hens. The loveliest of these is the beautiful and gracious Lady Pertelote. She holds the heart of Chaunticleer and shares in all his glories and all his problems.

http://www.cliffsnotes.com/WileyCDA/LitNote/The-Canterbury-T...