These first two women listed are unfaithful wives. We sort of understand Aphrodite's situation because she’s the goddess of love. Klytaimestra, on the other hand, is really bad news, as she not only cheated on her husband Agamemnon but killed him when he got back from war. Penelope, however, is a beacon of fidelity. These two adulteresses serve to highlight Penelope’s incredible virtue, which makes this a clear foil.
When Agamemnon was killed by his wife and her lover, son Orestes unleashed major vengeance on the miscreants.
Meanwhile, the suitors are wronging and dishonoring Odysseus, and Telemachos can't even bring himself to stand up to them. The parallel (if minor) story of Agamemnon's death shows us what a son should be doing to avenge his father—and why Telemachos is feeling so down on himself.
Klytaimestra is bad news: she not only cheated on her husband Agamemnon but killed him when he got back from war. Her super bad behavior highlights just how awesome Penelope is.