Good question. The answer, basically, is "It looks really cool". Of course, it also has to do with balance of style and variation of stroke. A sort of Fibonacci spectrum of focus. This is why it looks really cool, but ultimately my philosophy on painting is that if I could get the same effect with a camera, I could have saved myself three days of work and gotten about the result. Why on earth then, would I go and render the entire painting with the same consistent realistic effect? It would be a pointless waste of time. That is not to say there is no merit to realist painting, you might agree, that part also looks really cool. But to me, the balance of these aspects is a source of extraordinary beauty and a key goal in the endeavor of painting.