even if its flat a smoother surface has lets pits and valleys (the microscopic imperfections)

all these "holes" allow air to be trapped, even though are very small, it all adds up in preventing the best possible heat transferrence. the idea of thermal paste is to fill these gaps. anything is better at conducting heat than air but the thermal paste's properties is the reason it is so widely used.

but remember if you have a smooth finish you will need less paste