Why does a woodpecker peck wood?

A wood pecker clings to tree trunks propped up by it's stiff tail and rapidly hammers it's long, powerful beak against the tunk. The idea is to bore a hole for nesting or to get at insects and grubs which the woodpecker licks out with it's long worm-like tongue. The tip of the tongue is barbed do that the bird can harpoon tiny insects deep inside narrow tunnels.

img source: https://en.wikipedia.org/wiki/Woodpecker