I've always been fascinated by the concept of parallel universes and multiple earths. What if such places exist? What if there were another earth somewhere in which we all had counterparts who lived parallel lives with subtle differences?
1) This enables us to ponder the "what if" questions. What if you had chosen to travel the road not taken? What if you had a doppleganger somewhere who married the counterpart of the high school sweetheart you rejected? What if your doppleganger chose another career path? Is there another version of you somewhere who is living the dreams you abandoned? Or perhaps your doppleganger is not as successful or as happy.
2) What if you were born in another time? Maybe your doppleganger is older than you--born a generation or so earlier. The era of costumed heroes on Earth-Two began earlier than on Earth-One. So the Batman and Superman of Earth-Two had lived through the second World War, whereas their Earth-One counterparts had not yet been born.
Had the first crisis not ended the original multiverse, would the Earth-One Batman's life had followed the same script as his Earth-Two counterpart who did everything first? Might the Earth-One Batman eventually have married Selina Kyle? Would he have become Gotham Commissioner? Would he have had a grown daughter who fought crime as the Huntress? Might the Earth-One Batman, like his Earth-Two counterpart, have died at the hands of a criminal?
Might the Earth-One Superman have married Lois, as the post-crisis Superman did? A history of your older doppleganger's life may, or may not, be a prediction of your own future. An older doppleganger's history might also indicate how you would have fared if you had lived in an earlier era.
3) What if all fiction is real? In the multiverse this is possible. In the Flash of Two Worlds story it was explained that on Earth-One Jay Garrick was a comic book character and the inspiration of Barry Allen who called himself The Flash. The DC comics writers were in touch with Earth-Two through their dreams, meaning the stories they wrote really happened in another universe.
What if all fictional writer's imaginations were tuned in, unbeknownst to them, to parallel universes where their stories actually happened in reality? So while Wyatt Earp was Marshal of Dodge City in our old west, maybe there really was a Marshal Dillon on some other earth. And maybe there really is a Batman, a Tarzan, a Gomer Pyle, and a Cartwright family who lived on the Ponderosa. Cool!
4) What if you were born in another place? In the Elseworld's story, Red Son, Kal-El's spaceship landed on Soviet soil. Superman is an adopted Russian in this reality. In another universe, might your doppleganger be living in Germany, China, or Australia?
5) What if we gave in to our darker side? Johnny Thunder's Earth-One counterpart was a bad guy in Justice League of America # 37-38 (Recently reprinted in the trade Crisis on Multiple Earths, volume 1). On Earth-Three, the Justice League's dopplegangers are criminals. I'm glad DC brought the multiverse back. The possibilities are endless.