It has a very simple reason: to increase the area of the cortex - where the most important brain processes of our and other species take place - without the need to increase the cranial size.
Many vertebrate animals -among them man- owe their "intelligence" to the circumvolutions of their brain, which provide a very high and better neuronal activity. For example, the brain of the dolphin and that of the shark are similar in size, but that of the marine mammal has a greater number of circumvolutions, that is, it has a greater area for ''thinking''. A cat and a rabbit also have a brain of similar size, but the cat being a carnivore has a more refined way of life, so it needs greater intelligence, ie a cortex with more ridges and wrinkles'.
Animal dimensions are also an important factor. Rats and mice, despite their ''intelligent'' behaviour, do not possess brains with circumvolutions and, on the contrary, the cortexes of elephants and whales are more wrinkled even than that of man.
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