Art brushes are a powerful feature in Adobe Illustrator and can help reduce rather complicated steps in the creation of simple things. Think for instance what's needed to create a rainbow. It's basically a half circle of 7 colored lines of equal width. You'd think of creating each color line separately as a circle, but then you need to do the math to make sure each circle has the exact same width etc. This is a perfect example for the creation of an Art Brush.