Proportion, transparency, coherence, order, tranquility....these are the results of data that is properly layered and separated. An information graphic with layering and separation affords greater visual interpretation. The independent elements become more organized and discernible. The relationships between data points are more clearly defined. Chaos disappears. Tufte is essentially saying that layering and separation produces simplicity of form and shape and visually appealing data. He writes about the removal of "clutter" and "noise reduction" (Tufte 62). Layering and separation induce a calming effect on data. The data becomes easier to interpret and requires less time to understand. Data becomes less monotonous and repetitive. When the designer incorporates layering and separation some elements of the graphic become more salient and others become less. The result is a hierarchy of "visual weight," meaning the viewer is interpellated by particular data elements over others. Tufte references some ideas of Gestalt psychology, such as figure and ground, and I think this is effective. The layering and separation of data directs attention to what the designer is ultimately trying to convey in the graphic.
"Layering of data, often achieved by felicitous subtraction of weight, enhances representation of both data dimensionality and density on flatland. Usually this involves creating a hierarchy of visual effects, possibly matching an ordering of information content" (Tufte 60).