Why the CPU cooler is an essential device?

In the 286/386 CPU era, the number of transistors inside the CPU was not big due to the limited number of bytes and low frequency (from 6mhz to 12Mhz only). Therefore, the power consumption can be controlled under 3~4 Watts. By means of the free convection between CPU itself and air, the operating temperature could be successfully lowered. In those days, increasing the dissipation area by applying heatsinks to enhance the free convection would be good enough to solve the thermal problem. However, after the launch of 486 (50Mhz) CPU, the power consumption had climbed up to 8 Watts. The thermal problem then could not be effectively solved without forced convection which is created by CPU cooler, note examples Pentium 233 MHz (power consumption: 18Mhz) and AMD K6 233MHz (28.3 Watts).