It's something like an achievement of some sort. Hey, I had lived another year, and in that one year I've definitely experienced more, learned more, and hopefully grew wiser than what I was a year ago. There were good days, there were bad days, but dang I'd know that I would never trade those 365 days with anything. Think of it as a closure and a time to look back on what you have done, and the time to think where you should be going in this new year of your life.
That, and humans love to celebrate even the silliest of things