It's only to prevent damage, not concussions. They benefit from concussions and the scarring it causes. It's one more part of their "helmet".
Edit: IM WRONG THERE HAS BEEN BETTER RESEARCH AND I CANT FIND THE ARTICLE ON BRAIN SCARRING.
Edit 2: here's a better reason:
"The size and orientation of the bird’s brain safeguard it, the authors say. Even the strongest of wood pecks left the bird brains with less than 60% of the pressure needed to give a human brain a concussion."