In book VIII of The Odyssey by Robert Fitzgerald, why does Homer interrupt the narrative to have Demodokos sing about the love of Ares and Aphrodite?

Now to his harp the blinded minstrel sang
of Ares' dalliance with Aphrodite:
how hidden in Hephaistos' house they played
at love together, and the gifts of Ares,
dishonoring Hephaistos'bed __ and how
the word that wounds the heart came to the master from Helios, who had seen the two embrace; and when he learned it, Lord Hephaistos went with baleful calculation to his forge. (132-133)

The songs of the crier of Alcinous are fairly typical of what a bard might sing at a banquet, but they also serve a few other purposes for Homer's narrative.  First, they give the court at Alcinous' palace an idea of who this man might be (and an inkling of how high-born and famous he actually is).  That he weeps, surreptitiously and in manly fashion, at the song of Troy, they can surmise that he is perhaps a warrior, and had been at Troy on the Greek side.  He is also a cultivated man, they might conclude, since he appreciates music and can be moved by poetry.

When the story of Ares and Aphrodite is told, it hits very close to home for Odysseus.  Particularly when it is told that Aphrodite dallies with Ares while her husband is away (as Odysseus is away from Penelope) it causes Odysseus no end of grief.  He has all sorts of worries because of it -- that Penelope will be seduced by the cunning men who are in Ithaca, even against her will, that she will forget Odysseus, or that, worst of all, she would tire of waiting for Odysseus and choose another suitor.  This is a very upsetting story Odysseus, though he has been unfaithful to Penelope many times.

This sets up the Phaeceans to ask Odysseus about his story, and to bring their curiosity to a fever pitch.  Athena walks the streets telling of the interesting and handsome stranger in the king's house.  It is clear that the goddess wants Odysseus to tell his story -- it is as he tells his tales that we learn many of Odysseus' adventures, such as the Lotus Eaters and the Cyclops.   It is a neat poetic device by Homer, to make Odysseus sympathetic, and his audience ready for his story.

Source: Homer, The Odyssey. Robert Fitzgerald, trans.  New York: Vintage, 1990.