In certain kinds of (predominantly fantasy) stories, you have a situation where there is an overarching world structure that most people are familiar with, and then there is a sub-structure that some people are aware of. But how many people, and to what degree? In the case of HARRY POTTER, for instance, the muggles are all completely unaware of the magical side of our contemporary world. In the case of STAR WARS, everyone isn't aware of the Force and the Jedi, but most people are. In the case of THE DARK IS RISING, almost nobody knows of the Old Ones--barely more than a thousand people in the whole history of the world, I believe.
So, when setting up a fantastical world of your own, how do you handle it? From a storytelling standpoint, there are benefits and drawbacks to all of the above approaches. It ultimately depends on what kind of story you want to tell. In THE GUARDIAN I chose to have it so that everyone is aware of magic in general, and they consider it an everyday thing--but at the same time their understanding of magic is deeply flawed, and there is a whole other side to the Otherworld that most people aren't even aware of.
I used this setup for a number of thematic reasons, but it has the drawback of raising a lot of questions regarding why people don't figure it out. That's something that is more addressed in my second book, but is hinted at in THE GUARDIAN--basically it is noted that certain aspects of the Otherworld can be perceived in multiple ways, and a central controlling body (the Ministry of Colonization) guides people's perceptions to the conclusion that is less alarming--as if most people need the help.
In ALDEN RIDGE, I also have secrets that only a few people know, but people's general understanding of what is happening in their post-apocalyptic world is not that far off. They may not know why things are the way they are, and they certainly don't know about some of the darkest things that lurk in their world, but in general their reality isn't far off. This is more the STAR WARS level of public knowledge, whereas THE GUARDIAN is something more like the HARRY POTTER level of public knowledge (though the similarities between these works mostly end there).
A story where the public generally knows about the dark/fantastical things is easier to write in some ways, because it makes your character freer to act and seek help, but it also makes it harder to write in other ways, because you have to do worldbuilding twice--once for people's public perceptions (all that is "general" knowledge), and once for the special things that the protagonist learns. I think that's pretty universal in most stories--whatever the scale, the protagonist is going to learn some things that few other people know. It's really just a question of degree: how much of what is mystical and dark is generally unknown, and how much is known?
And I think that boils down to two questions that are often hard to answer until you are a ways into writing or planning your story: 1) Does either approach trap you into any "rules" that contradict other plot elements you want to include, or otherwise force you down any paths you want to go down; 2) Does either approach offer you a more powerful/exciting/emotional angle that you just really want to have?
To me, secrets are an important aspect of worldbuilding in my own writing, because that's just the sort of stories that I tell. I like the element of mystery, though it's certainly not the only way to tell a story with large-scale fantastical elements. It's all about finding your own style, and making sure that your worldbuilding supports the kind of stories that you most want to tell.