It's more to create a field of depth than anything. When most people think of 3D, they think of the red and blue glasses kind where the only thing possible is to have stuff pop out at you. Digital 3D does so much more than that, and while it can have stuff pop out at you, that's not the name of its game. It's honestly more of a side-feature.
As for the "foreground images", this is part of the technology. The 3DS literally splits the image on-screen via some sort of lighting magicks, causing your eyes to have to adjust (which causes eye strain for some). This is what creates the 3D effect, as your eyes are now seeing the on-screen image from two different angles. When the background of an area is dark, this is evident, as it's possible to see translucent images of objects to the rights and lefts of them. Depending on how high you have the 3D effect amped up will depend on how far to the sides they are. I'm not quite sure why it takes dark surfaces to see this -- or why it's visible at all -- but that's what I've observed with my time on the 3DS. Didn't take much observation, really. I noticed it pretty quickly when I played through OoT 3D for the first time.