The Saguaro cactus [Carnegiea gigantea] takes about 40 years before it forms its first branch. That also is the length of the wait before the plant's first buds. As a result, the plant's hallmark huge flower and tasty fruit likewise don't show up until the giant cactus is about one-fifth of the way through its 200 year life expectancy.
Plants need pretty much what any living organism needs -- water, food (nutrients), shelter, light, warmth, space to grow and protection from enemies (including competitors for these resources).
In order to understand why desert plants have cuticles (cutin) you first have to know what cutin is, it's function, and why some plants have it. Then you can begin to answer why plants specialized for desert climate have cutin.
Cutin is a fatty, wax-like substance secreted by a plant's dermal tissue system. The dermal tissue is also known as the epidermis which is the outermost layer of cells on all young plant organs. If you consider all of this "waxy stuff" as a whole it is termed "cuticle,"similar to the cuticle above your nails. As you can imagine, wax is insoluble in water and therefore "water-proofs" the skin of the plant. In other words, it dictates how much water is lost through the epidermal cells by evaporation, ensures water does not enter, and also keeps other molecules from entering. The cuticle also protects again UV radiation.
Regarding plant's evolutionary history, the adaptation of cutin provided a way for marine plant life to move onto land. Suppose if you were a marine plant and your entire body was surrounded in water, you would rely less on gaseous exchange and evolving structures or mechanisms to trap water within you. If you move onto land and are in hot, dry, desert climates water is scarce and you must adapt to survive. This is why desert plants have cutin.
I hope that helps! I am a biology major and am currently taking a botany course.
== == typically where there is alot of water, like the rain forest dont no