When I watch time lapses of Photoshop (and the like) artwork, I always ask myself why they paint a huge area over the picture over and over again (like layers).
As reference watch the first seconds in this video.
The artist paints a large chunk of black over the picture (the fine black drawing itself is always on top).
Is this to provide more contrast? Is this like a backup? Painting a big area implies that the layers beyond are finished?
Later the black is overpainted again.
Since I am not a painter/drawer/designer etc, I have no clue what this is supposed to do/help with. I used paint to make some signatures in discussion boards but those where only patterns and filter.