Zeus is up to his usual tricks again in this story: seducing a young maiden, causing her destruction, getting off scot-free... the whole shebang. More than anything else, the king of the gods seems to be present in this myth to play his role as the big daddy in the sky. Basically, any great hero or god needed a godly father. Otherwise, how could said hero or god be so great, right? Since Dionysus was one of the most popular and widespread gods, nobody but the most powerful of all gods could have been his dad.