Seriously, cosmologies and histories are all well and good, but the ultimate focus of the GM needs to be "what do the PCs do".
For example, in my GURPS campaign long, long ago, I decided that the world had more than one moon; the waxing and waning of each would affect magic. However, keeping track of the calendar soon became a chore (i.e. I lost track), so that part I just tossed out.
Even in the campaign I've been plotting forever, I have these grand ideas ... but unless I can work players into court intrigues, religious movements, the Goblin Problem, or the multi-layered strangeness of the City of Silence, it's all for nothing.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)