It'd be a chance for an aging Luke to learn more about the mother he never knew. Anakin's ghost could tell Luke about Padme and what she was like, provided he hadn't already told both of his kids about their mother in the intervening years between JEDI and Episode VII. "Father...tell me more about my mother." "She was a kind and a beautiful woman, Luke. And I loved her so much that I gave up everything...including my soul...because I wanted to save her..." Something like that.