Octopuses have this sort of setup, with a smaller brain to direct movement in each of their tentacles, but say we had a humanoid with these extra brains specifically for their arms and hands. How would they benefit from this setup?
I don't remember where I heard this, but I seem to remember that certain actions of the human body, i.e. reflexes, are actually controlled by nerve clusters along the nerves in the body to reduce latency between sensation and reaction. So, the benefits would be faster reflexes.