Why Does A Cactus Have Spines On Its Stem?

A cactus typically contains precious water which is extremely precious in arid locales.  Thirsty animals are thwarted from attempting to break open such a plant if it has protective spines.  Another interesting aspect of cactus is their wavy bellow like exterior.  During dry periods the cactus contracts as it uses up its reserve of water, these bellows allow the cactus to reduce its circumference.  However when the rains return, the cactus absorbs massive quantities of water, its girth expands and the wavy surface flattens like an outstretched accordion.

To prevent animals from eating them.