Answer to Question 1
Shades, shadows and reflections enhance a drawing with depth and realism, making it easier to read. Precise construction is typically not done on interior design presentation drawings due to time constraints and the visual clutter they may bring. However, precise construction must be understood for simplified shadows, which are used on presentation drawings, to be competently drawn.
When done well, shades and shadows may make orthographic drawings so easy for the client to read that the need for perspective drawings may be eliminated.
Answer to Question 2
They contain a colorless solvent that breaks down pigment, enabling you to obtain multiple values from one marker. For ex., to get a lighter value, use a marker and scribble it onto tracing paper (it's hard, nonporous surface is conducive to this process), then use a blender marker to rub over the scribbles to pick up the color, then dab it onto the paper and create a lighter value.
Another way is to layer a color on top of itself after the first layer dries for a darker value of that color. Three values of a marker can be obtained by layering, and a fourth value by applying the marker on the opposite side of the paper, which also takes away paper reflectivity. A fifth value can be obtained with a blender marker. When layering, start with the lightest colors and layer dark ones over them.
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