It's like a scalp massage!
I had a plastic brush with the little round-ended bristles. These often broke in my thick hair (and actually the brush handle broke on one!) and sometimes didn't feel like the bristles even reached my scalp.
I don't have this problem any longer with my new large bamboo brush. It feels solid so it won't break in my hair, and the bristles actually feel like they're getting in there properly. It feels like it helps with getting the blood flow to your scalp.
It also has a lovely smoothing effect on your hair, and anti-static effect.
Totally recommend for anyone with long, thick hair.