This does two interesting things:
- It will blur according to the values of a channel (good for depth of field / defocus effects). A black channel value will return no blur, white will return full blur and mid grey will return mid blur.
- It will add realistic wikipedia:bokeh effects and highlights.
Though there is no other way to mask a blur in PS I find it more convenient to construct defocus / depth of field by chopping up the image into layers and blurring each layer to a different amount. The Box Blur is an imperfect but reasonable substitute for a basic defocus. The highlight blowouts that are a feature of a real defocus can be faked by duplicating the un-blurred image and then doing an extreme Curves highlights clamp (i.e. whiten the highest points and blacken everything else). This can be set to screen over the Box Blurred result.