Illustration is the commercial equivalent of doing fine art on a commission basis.
Norman Rockwell accepted over 300 commissions to create paintings for the covers of the Saturday Evening Post that have become icons of American culture. One is a fine artist and one is an illustrator, but you'd have trouble defining the line that separates them. It's inexpensive and prolific and it can be useful to the graphic artist to rough out an idea. Hear me now, clipart is NOT the stuff you want to use in building an identity for your business. Because it is art in the public domain - available for anyone who wishes to use it, however they see fit.