This part is specifically designed to make the moire/tree ring effect, caused by inaccurate microstepping or otherwise insufficient horizontal resolution, visible. Use this to test for it so that you can recalibrate to get rid of it. You probably won't need to worry about it unless you are trying to print layers .1mm or smaller.

You don't get the full effect on the round pieces from the pictures, but check out the full-size (which is also upside-down for some reason) to get a better look: http://thingiverse-production.s3.amazonaws.com/assets/7e/1e/bb/0e/66/Photo_Jun_12_12_40_03.jpg