I believe they are common because people believe that is where we could be headed. Will we run society into the ground, or will we prosper? Will the society be a "Divergent" or "Giver" society or will it exist as it does today? Since we have no way of knowing, we pretend we do.
It is D: to brainstorm by putting thoughts onto paper without regard for spelling or mechanics. the reason being is nobody but yourself is going to look at the free write.
Clustering is a type of prewriting that allows you to explore many ideas as soon as they occur to you. Like brainstorming or free associating, clustering allows you to begin without clear ideas.