I'm about to start my third campaign in my homebrew setting, having run one campaign 1-20, and still in the process of another (currently at 15th, IIRC). When I first put the basics of the setting together, I was putting things (or taking things out) mostly according to what kind of setting I wanted to run in; as I've added things--whether in the process of setting up campaigns or working out adventures or just being curious about the world--I've started adding things I am, at best, reluctant to run, but which seem as though they need to be in the world (this is mostly cultures I don't feel as though I know well enough to center the game around) and I've also started adding things with the specific idea the players/characters would--or at least could--interact with them interestingly (though these tend to be things that get added in ongoing campaigns, when I have a sense of what a given party is interacting with). I don't see a lot of point in adding things to the world the PCs will never know about, let alone interact with--at least not with the players won't ever know/see/touch/feel/understand this being the point of the thing. I also make a bit of a point o being generous with world information, because most of it is stuff the characters would know and I want the players to be able to use it to make reasonable, rational-ish decisions for their characters.