Illustration is functional, it tells stories, builds adorable or ugly figures, and decorates texts.
As an illustrator, what I concerned for my artworks are two aspects: making believe and aesthetic. Depicting fantasy contents by using representational drawing skills is a way that inviting viewers to believe the possivilities that the world I created might be truly exist in somewhere, somehow.
Like James Wood write in How Fiction Works: 'You can ask a audience to believe the impossible, but not the improbable.'
On the other hand, aesthetic is also important because that is how an artwork can catch audiences' eyes at the first time and then let them have affectivity to the fantasy world/creature.