2 Answers

Some frogs skin can be fatal if touched. Its a defence mechanism.

For all the other frogs, the skin is used for the same reason YOU use your skin.

IMAGES OF POISONUS FROGS

http://www.google.com/search?q=poisonous+frogs&hl=en&qscrl=1&nord=1&rlz=1T4GGLL_enUS354US355&biw=1280&bih=825&site=webhp&prmd=ivns&tbm=isch&tbo=u&source=univ&sa=X&ei=ipZXTsbMHePY0QHUz_ycDA&sqi=2&ved=0CCEQsAQ

http://www.thefrog.org/biology/skin/skin.htm  The Frog says frog skin is lycra like and it protects them from injury and disease. It often is a camoflage or hiding mechanism for the amphibian. When frogs shed their skin, they eat it for nutrition. Their skin is water permeable which means it lets water in and out of the frogs body--they do not drink water by mouth, they use this permeability to get the water they need. Capillaries suck the water into the frog. Skin is sometimes a source of poison or other noxious substance to repel predators. Skin colors often alert predators when a frog is poisonous and to be avoided as food. Skin color changes on some frogs to help regulate temperature--being cold-blooded, when skin is light they stay cooler and when dark they get warmer.