The reason that this is important is that we put in our picture that which is familiar as the context for the main idea. It is familiar because gravity makes it so. Things in nature are mainly perpendicular (trees, live people) or parallel (ceilings, dead people).
Having established familiarity, we can move on to the main event without conflict.
To do otherwise is to lose control of the composition. It becomes a chaos. And I do grant that at times there is a need to represent chaos ... then you avoid such controlling parallels and perpendiculars to such extent as to provoke the aforementioned chaos.
Here is the definitive work.