If you want to illustrate stories or book covers, draw storyboards or design conceptually for games or movies, it's vital to grasp the foundations of representational art. And with most stories you'll come across that involve people, it's important to understand how to draw the human figure, both in a static pose and in action.
There are several techniques for drawing the human body, all leading to a similar goal - a three-dimensional, realistic figure. While it's not necessary to be an expert to produce illustrations, the more knowledge you have, the easier it will be to solve problems and reach clear-cut solutions for any drawing.
What you lack in your foundations will show up in your work - that is, a lack of understanding of certain principles will be all too apparent in your finished piece. In other words, an artist's style can be reflective of his lack of understanding just as much as it can be a showcase for the total sum of his knowledge.