Ships, houses, mills... in one word everything that is made by people must stand upright and be painted with care. This is actually a good presentation compared to other, less symmetrical things, like the trees, skies, etc. It doesn't create the painting, but it certainly strengthen the illusion. It's just like somebody who is neatly dressed, but whose tie is coming off. The windows of a house must be straight, a mill in a pure construction, the blades well-positioned in perspective.