MasterGwydion
Emperor Mongoose
I don't run adventures. I run sandbox campaigns. Generally, my players pick a Subsector or (more rarely) a Sector and stay within those bounds. The less detail that exists about what is in those areas, the more I have to do in order to create a seamless campaign world for My players. Having more details provides more opportunities for things that are happening around the periphery of the players' attention. It allows Me to use ideas that some wonderful writers have included in the setting, that wouldn't have occurred to Me otherwise. My favorite place to run games in in the Trojan Reach exactly because there is so much written about that sector and its surroundings, and it is outside of the 3I.I agree 100%. The need to remove all mystery and unknown from a setting is a mistake in my opinion. And no, a GM does not need to know 100% of everything in a setting in order to run a great game either.
I can run a mystery game in a city that all of My players live in and can google every detail possible about the city. No fictional setting will or can ever have that kind of detail, but a well-run campaign can still be very interesting and still have tons of surprises.
Edit: As an example. If I need a specific set of circumstances for an idea for a story? I grab the books and see where that story could best fit. The more detail I have, the easier it is to find a place that it fits. After that, I make up whatever additional levels of detail are needed for the story.