I think a lot of these experiments are more about the challenge; Just thinking ‘I wonder if I could do that using existing tools’ and might not be of much use in the end. 
I can see uses for this one though - like making watercolour texture effects, bokeh style effects, or natural textures - such as lichen spots on rocks etc.
The brush in the video has hue variation, but I switched it off for the demo. It works better with grainier brushes - imitating the effect you get with some watercolours.