answer:The simple answer is that the folding allows the cerebral cortex (the 2–4mm thick outer layer of the brain) to occupy a greater surface area. The inside of the skull is about 600 sq/in, but the surface area of the cortex is 1600–2000 sq/in; 2/3 of the surface is actually down in those folds. Humans need an awful lot of cortex, because this is where our perceptual model of our world resides, as well as our centers of abstract thought, language and planning, along with many lower-level sensory functions. What actually causes the folding is far more complex. Here are some of the current hypotheses: “Various hypotheses were proposed for the development of convolutions, such as active growth, pressure and friction of expanding cortex tangentially against the skull or underlying brain structures, and mechanical bulging from unequal regional expansion. Other concepts suggested that cortical buckling results from differential laminar growth, or that convolutions are shaped through attached axonal fibers. More specifically, it has been proposed, but not yet rigorously tested, that the 3-D shape of the brain reflects the viscoelastic tension exerted by axonal fibers. According to this hypothesis, global competition of mechanical forces results in the formation of gyri between densely linked regions, and sulci between weakly connected or unconnected regions. The axonal tension hypothesis is particularly attractive, since it implies that the characteristic cortical morphology arises automatically from the interconnections of different cortical areas, without the need for individual specification of convolutions. Moreover, cortical folding through axonal tension implicitly achieves a desirable reduction in the volume of cortical fibers.” (source) To translate that last “particularly attractive” bit, it may be that regions of the cortex that are rich in the connecting nerve fibers called axons end up forming the “gyri”, or ridges, whereas the less connected areas form the “sulci”, or creases.