Illustration is a visual language. One of illustrator’s functions is to take an idea or a concept and transfer it into visual information regardless of the media.
This is especially true for editorial illustration, where artists are commotioned to do illustrations for articles or covers of magazines. Some illustrators, like Norman Rockwell, did/do extensive and very detailed studies of their subjects and themes to execute out of this world illustrations that can easily stand alone as masterpieces in a gallery.
Illustrations can also follow function. For example, technical illustration’s function is to serve as a guide (ikea manuals for their furniture) or pattern illustration for industry (wallpaper, fabrics, interior design, etc) or medical illustration for education.
Sequential illustrations can be used for entertainment, like comic books, graphic novels and animation.