Examples of such art are:
This piece, while utterly bizarre at first glance, could have come from a dream of sorts as barely anything makes sense in a dream. Architecture in concept art doesn't necessarily have to mirror exactly that of real life, so the concept artist can distort shapes to their liking without having to worry about things like the laws of physics.
The scale of this castle is utterly immense, possible only through the fantasy world. Concept art plays with the ideas of the impossible and exaggerates it a hundredfold.
Character design is also another major subject in this form of art. This one in particular explores the niche theme of steam-punk robotics, which also explores the themes of both sci-fi and fantasy.
Avatar's stunning CGI world 'Pandora' took the film world by storm with its otherworldly beauty and vast sense of scale. The Hallelujah Mountains in particular looked like they belonged right in a piece of concept art, which is, of course, where they originally came from.
Concept art is the incarnation of ideas put onto paper by artists whose minds and imaginations expand into vast universes.