Air cooling is, as noted by fussnfeathers, sufficient for most systems and users. But air cooling depends on air movement within the case, to ensure temperatures stay within limits. So fans must be added to force air through the case.. not just those that circulate the inner air mass. Best is a ratio of 1.5:1 for incoming air to exiting air.. to avoid starving the PSU of its cooling needs. A case fan blowing toward the heat-generating devices is best.
Water cooling, on the other side of the coin, is more efficient, actually moving unwanted heat from one zone to another.. but is more complicated and expensive. And requires more maintenance.
F@H.. to solve mankind's maladies.. in our lifetimes!