Why are clothes important to you? Do you think that trees will enjoy hanging around naked?
Because trees suck at biting. Also, the bark contains the phloem which is the vital tissue of a plant that transports the organic compounds produced in the photosynthesizing parts to the non-photosynthesizing parts. It also protects the inner wood (xylem) from prying eyes so it can hatch its evil schemes without interference. What, you didn't know trees were evil? That proves bark works.