What is the story of Aegisthus, Agamemnon, Clytemnestra, and Orestes? Why is it repeated so many times in The Odyssey?

Aegisthus had an affair with Clytemnestra, and they both killed Agamemnon - Clytemnestra's husband - during a banquet. Agamemnon is the commander-in-chief of the Greek forces when they invaded Troy. Orestes is the son of Agamemnon who sought revenge by killing Aegisthus. Homer mentioned the story a lot of times in order to provide a series of happenings regarding Oddyseus and his family after Agamemnon was slain.