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.