You have to write a paper, or copyedit one, and you have a heap of style manuals in front of you. Which one do you use? Are they consistent? Is there a difference between them? Is it all arbitrary?
As you work on your text, it stares up at you, glaring with its colons, commas, and parenthetical citations. Where do they go? Are the lines single or double-spaced? How wide are the margins supposed to be?