I do it as sort of a "closing the book" on the previous year. I don't mind looking back on my mistakes and reminding myself that they are the reason I am the way I am today, but stashing them on a shelf is a lot better than constantly telling myself I used to be an idiot.

I used to do it because I posted my Sketch of the Day project here, and wanted folks to buy the CDs (thus, making the art exclusive to the CDs), but it became a tradition separate from that :)