Basic physics.
I would leave it there, but . . .
If you apply five dots of paste (one at each corner, one in center), then, where they come into contact as you're pressing on the heatsink, you will develop air bubbles. Not big bubbles, but still enough to interfere with efficient heat transfer. The goal is to fill microscopic pores and defects in the metal surfaces that will be making contact. Air is a very poor conductor of heat; copper and aluminum are very good conductors. Therefore, you want as perfect a contact as possible. The paste facilitates this. So, the best option is to apply a small amount in the center of the heatcap. As you apply the heatsink straight down, the pressure applied will force the paste to flow outward from the center, filling pores as it goes. When finished, the layer of paste will be extremely thin. This is what you want, and it minimizes air pockets. If you spread it out first, you will trap air as you do this. Applying the heatsink will just keep it there.
Now, we're not talking extremes here, so it's not going to be extraordinarily critical how you do it. This is why this topic continues to be discussed and debated on every tech forum on the internet.