What is the purpose of the story of Hercules and Cacus in the Aeneid book 8?

Like many of the details in the poem, this story provided back-histories for Roman traditions and notable Roman citizens. Overall the point of the story was to establish why there was a Roman cult that worshipped Hercules, but names of priests who served at the altar of Hercules were also supposed to bring to mind well-known current-day Roman citizens and "prove" the legitimacy of their nobility by tracing their lineage back either to the earliest days of the tradition, or for many , by tracing it all the way back to Troy.