In book XI of the Odyssey,how does the visit to the underworld act as both a plot device and a thematic tool?

In Book XI, Odysseus' time spent in the underworld moves the plot by affording the reader or audience additional information on the history that affects the action. This book also casts light on four other defining themes in the poem: fidelity, obeisance to the gods, temptation, and endurance.