The first four books of the Odyssey are often referred to as the Telemachy because they primarily deal with Odysseus’ son Telemachus… but why include this section? What does it add to the epic? Or does it take away?
Oedipus Rex has a sequel about his daughter and her traveling with her father who was blind: thus, her perils as an unprotected woman. It is a tradition to foster the sequel as a consequence of the hero’s actions.
I suspect that it was simply a sales ploy, why not advertise your next play in the first. Many authors follow this idea, it does not have to be new.