Answer:
Bark comprises all the tissues surrounding the xylem in woody plants. Broadly, bark includes an inner and mostly living region and an outer, dead one. Of all plant structures, bark has the most complex anatomical structure and ontogenetic origin involving two (and often three) different meristems.
As to why, the bark adds layers to the inside ring of the vascular cambium, the outside is pushed outward. The cells die off, and the fibrous matrix of cellulose and lignin molecules remain. This hard structure forms the bark, and protects the tree or plant from many forms of damage.