Why does cactus have so many thorns or spines?

Reasons for cactus having many thorns or spines:

1. The Cactus is a desert plant that grows in regions that have less availability of water.
2. The leaves of the cactus is reduced to form thorns or spines which have many functions.
3. The spines are needle-like and have significantly less surface area as compared to a normal leaf, so the rate of transpiration is less.
4. This way, cactus can reduce the loss of excess water and survive well in hot and dry environments.
5. Spines have pointed, ends, and some cactus bear poisonous spines, too, which provide protection to the plant.