On page 53 Tufte introduces the topic saying that to avoid confusion and clutter separation and layering reduce noise and enriche content of displays. These design devices are used to distinguish various aspects of the data so that they don't become noise and clutter.
"Layering of data, often achieved by felicitous subtraction of weight, enhances representation of both data dimensionality and density on flatland."
Tufte shoes that signal to noise ratio is critical to reduce viewer fatigue(pg 62) and improve accuracy of readings.
"Clarity is not everything, but there is little without it." Tufte pg 62
We are so tempted to fill up empty space. Horror vacui or fear of empty spaces is worth overcoming. Tufte makes his point about increasing the signal to noise ratio with layering and separation.