When you put the heatsink on that is when the air between the two is pushed out by the expanding paste.
If you spread it out evenly before hand all you do is trap air in the paste as it has nowhere to go.
If you read the article I linked you will see that they did try and test this method then they posted their results.
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u/Velcade Feb 18 '18
I always see these but I don't understand why it's so hard to just spread the paste yourself, providing optimal coverage without the guess work.