The Book of Genesis is a real grab-bag of stories, some adapted from cultures nearby to the Israelites, but the story of the Garden of Eden seems to be a myth-of-origin story, like “How the leopard got its spots.” Eating from one tree but not the other, then being expelled from Paradise explained why life was so difficult; and why people got sick and died. Probably it gave those ancient people some comfort, and added some meaning to life and death.
(This post was written while listening to Nickelback's The Long Road album, which shows how much I care.)