A - The reason being is because of atmospheric perspective, it's most commonly used and more well known with point perspective that gives drawings, paintings, or otherwise 2D mediums a 3D feeling.
Spatial perspective is usually used within natural / organic compositions because of the foggy or distant feeling it can give.
Interest because of how many different techniques you can accomplish with perspective such as one point, two point, or three point perspective. Such an example would be in architectural mock sketches of the finished building or project.
Depth, as stated before depth and space are both hand and hand. Depth gives the feeling of something coming alive or making a circle into a sphere with shadows by using light, creating layers of rock on a cliff face instead of a sheer drop, moreover; detail.
