Not just drawing
Illustration design can seem like an unnecessary tool for graphic design. Most of the time you can use a variety of other design elements to get your point across. Most artists begin with an actual pen-to-paper thumbnail sketch to help the creative process.
It's about mastering all of the elements available, and integrating them together to create the best design. This can also be beneficial for a client from a budget perspective. A designer that has illustration experience can sometimes save a hiring of an additional person and leaves project options open