Expanding objects enables you to divide a single object into multiple objects that make up its appearance. For example, if you expand a simple object, such as a circle with a solid-color fill and a stroke, the fill and the stroke each become a discrete object. If you expand more complex artwork, such as an object with a pattern fill, the pattern is divided into all of the distinct paths that created it.
You typically expand an object when you want to modify the appearance attributes and other properties of specific elements within it. In addition, expanding objects may be helpful when you want to use an object that is native to Illustrator (such as a mesh object) in a different application that doesn’t recognize the object.