The scientist in me wants to answer the thought that "They probably do it for the same reason human beings compose sonatas. Both humans and birds get bored with monotony. And to counter monotony you always have to do something new to keep the brain aroused."

Remember, we're talking about bird brains here. Chances are it's simply Mother Nature providing the variety (aka diversity) that gives evolution a moving target.
But I wonder, if the purpose of the serenade isn't to promote ovulation in the lady on the balcony?