Interaction Of The Heat Spreader And Heat Sink
A microscope will show you that neither the surface of a heat spreader nor the surface of a heat sink are really smooth. What looks even to the bare eye is full of pits and grooves.
When you press both surfaces together, only parts of the metal touch each other. Without a thermal compound, air fills the gaps. But air is a bad heat conductor. It's more of an insulator, actually. Thus, without thermal paste, much of the engineering that goes into heat spreaders and CPU coolers is wasted, as heat is only conducted where the metal surfaces touch.
Heat-Conducting Materials to the Rescue! Pastes and Pads
Clearly, the insulating air needs to be displaced by some thermal compound. Obviously, any thermal paste, pad, or liquid metal will conduct heat less effectively than the two metal surfaces involved. So, you want the application to be thin enough to not impose a lot of thermal resistance, but thick enough to overcome the surface imperfections of the heat spreader and sink.