Temperature is crucial
The chemical processes in animal cells require specific temperatures to function optimally, and in many animals, the temperature interval is very narrow, so they have developed special methods for regulating body temperature.
Warm-blooded animals such as mammals are able to generate heat in their bodies themselves and maintain a constant temperature. The major disadvantage of this method is that the heat generation requires lots of energy. So, warm-blooded animals need to eat a lot, and they must be insulated in order not to lose too much heat. Hence, they have fur, feathers, or insulating layers of blubber. Cold-blooded animals cannot generate heat in their bodies themselves. Instead, they have to absorb heat from the surroundings such as from sunlight.
This means that they do not need to eat as much, and so, they can live in surroundings with much less food such as deserts. They usually require 1/10 of the food that a warm-blooded animal of the same size needs. On the other hand, cold-blooded animals must spend a major part of their time moving back and forth from sunlight to shade, when they get too hot, and from shade to sunlight, when they get too cold. Among the typical coldblooded animals, you will find reptiles.