Why does The Odyssey's start fit the description for medias res?

The story of The Odyssey starts "in medias res" ("in the middle of things"), relating in brief exposition the background before jumping into the present narrative. Homer's contemporary audience would have already been familiar with the story of The Iliad, whose events precede The Odyssey, so there is no need to waste time reminding them of it. Remember that the poem was delivered orally, so an audience member could not skip through the opening pages at his leisure. More importantly, Homer kick-starts the narrative engine, and already in Book I we see various plot machinations at work and an emphasis placed on internal stories, which often have a thematic impact on the major story. For instance, the story of Agamemnon parallels that of Odysseus.

The odyssey begins In Meadows Res, or in the middle. Why would an author choose to start in the middle of the story and use a flashback? What effect does it have upon the reader

Homer repeatedly mentions Agamemnon’s death at the hands of Aigisthos. Explain how this story is relevant to both Telemachus and odyssey