Artists adopt this technique for valid reasons. Firstly, let's define 'thin' and 'fat.'
Thin – implies applying paint thinly, achieved by either mixing it with a medium or scrubbing it onto the surface for a delicate application.
Fat – refers to impasto paint, straight from the tube and applied in bold, thick chunks.
Why start thin?
I have two significant reasons and other minor ones.
Laying down a 'fat' paint area limits subsequent actions. Applying fat on fat results in muddy outcomes when painting wet on wet.
Starting with a 'thin' area sets the stage for contrasting fat-painted details on top. The subdued, thin paint complements the bold, attention-grabbing fat paint.
Typically, early details should be placed thinly, gradually increasing thickness as you progress, concluding with bold highlights that mimic a thrown-on appearance.
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Ron