The best designed heatsinks can significantly under-perform if there is too much contact resistance between them and the device. The purpose of thermal interface material is to minimize contact resistance and maximize overall thermal conductivity from the device to the thermal solution. TIM2 is used for lidded devices, and TIM1.5 for lidless devices.
It is a very important aspect of any thermal design, and one that deserves some time and thought so as to not compromise the overall thermal integrity of the solution. Figure 1 shows a cross section of a lidless and lidded package along with the different thermal interface material (TIM) layers.