Some trees in hot dry climates have needles like leaves, which have few stomata, thick waxy curticle. why?

Im going to use a cactus as an example. A cactus is a plant that lives in hot, dry climates, which has needle like leaves to protect itself from animals searching for water. It has a thick waxy curticle to make it hard for an animal's teeth to pierce the plant and to help keep the water in the plant (the heat makes it easier for the plants water to be used up at a quicker pace).

From a high concentration to a low concentration.