The Outer member defines object ownership, and ultimatly the package the object will be saved in, with one exception: external packages. Objects car define a package to be saved in, even if their Outer is saved in another package. This is used for external actors and other object types.
Also, the Outer chain is used to determine if an object will be duplicated or not during duplication; when duplicating a World, all objects rooted in the World through their Outer chain will also be duplicated. For example, during World duplication, a normal texture asset will not be duplicated, but if that texture is Outered to an actor, it will be duplicated.