This is my theory based solely on my ignorance and lack of intelligence. I have both, I use both. I do NOT have any super high end badgers. Nor, badger hair brushes. I do have one smart German Shepherd, not sure if that helps matters.
Anyway, because of the stiffness, of the boar, not only do you get more backbone and chance of lather with a soap this helps to exfoliate your skin better.
It's like comparing cotton to loofah. Which one will scrub away those dead cells?