What can thermal paste be used for?
Although the metal base of the CPU / GPU cooler and the surface of the CPU / GPU appear smooth to the naked eye, these metal plates have microscopic voids that can cause poor heat transfer.
Due to such imperfections, the two surfaces are not in full contact, and the thermal paste can fill the gaps to conduct heat more efficiently.
In short, thermal paste helps your CPU / GPU cooler to function properly, while a cooler CPU / GPU reduces potential performance issues like throttling.