Why do we attribute value to "round figures"?

Today, my simple little science blog on Google+ crossed 15,000 followers, and with it, I initially felt I nice jab of contentment, pride, and other usual emotions that accompany achievement.
Then I wondered a bit. Why does this feel like an accomplishment? Why now? Why not a few days ago, when I had 100 less people? How did the 0.6% increase make such a dramatic difference?

Does it provide a reduction in complexity, thereby making more attractive for the brain to remember? If so, why?