Whats up with smerfing in games????

The one thing that bothers me the most when gaming are smurfs. They are the Bain of my existence. What possess you to make a smurf account when you are a high level? What do you get out of it? Is it cause you wanna feel good about your skill level? Or is it sumthing like bullying/trolling?. I think it's just mean. :/