Shared Layer Styles allow you to attribute specific styling to an element. For example if you know that a generic button is going to be a Blue, but don't know what shade of blue yet, attaching the fill to a Layer Style will allow you to update on the fly if need be, everywhere in your document that shares that similar Layer Style. Think about Layer Styles as "locking in" a current selection of styling to be used in other areas on your UI, but still have the ability to manipulate those values everywhere. Using Layer Styles improves your workflow and efficiency.