Why's it slightly noisy? It never resolves to clean when the camera's still, so it doesn't seem to be MC noise for something like ambient occlusion or raytracing/pathtracing, so I assume it's just a 2D effect on top of the image for look?
Plus someone probably decided it would look cool...
Perhaps it just doesn't stack the renders - if it did, changing view would decrease the image quality back to low samples, maybe that's the reason...