What do you need thermal paste for?
The surface of a heatsink is never completely flat. Its surface is covered with microscopic abrasions caused by the manufacturing. Direct contact would create air spaces that have an insulating effect on heat transfer. Therefore a filler material with very good thermal conductivity is required to compensate for the unevenness between the cooler and the chip and to fundamentally improve the heat transfer.https://www.arctic.ac/en/What-do-you-need-thermal-paste-for