Outside of Star Wars, a great example (and personal hero of mine) is Strider from the Lord of the Rings. I thought he was awesome as a Ranger—but the whole 'lost king of Gondor' thing blew me away.
I imagine a lot of people feel the same way. I mean, who wouldn't love to suddenly discover that they're the long-lost heir to some forgotten kingdom or the descendent of some great historical figure? Well...maybe as long as that figure wasn't like... Hitler or Stalin or Carrot Top, but you get my point, I think.
I've used this tactic before in games. For instance, in my Star Wars campaign, Adren discovered that she was the long lost grand daughter of Lady Santhe (a corporate/industrial magnate). In another campaign, my friend Philip's character (Mason) discovered he was the long lost heir of a noble house in the Tapani Cluster. Both of these incidents played out very dramatically and added a lot of fun to the game (for me and the characters). It makes me wish I had tried it earlier in my gaming career.
I imagine that I didn't incorporate secret backgrounds because I was still kind of locked into the 'simple hero' mold of most D&D games—where characters start out as little more than adventurous peasants and have to build from there. Not that there's anything at all wrong with this approach. In fact, the 'average guy done good' is a great concept—most of my Star Wars PCs fall into this category. Oman was a simple bounty-hunter who rose to become Mandalore. Hugganut was a street punk who now owns a luxury gambling resort world. Arianne was the child of a couple of essentially homeless rebel activists who rose to be a high-ranking officer and Jedi. So you see, Secret backgrounds aren't NECESSARY, but that doesn't keep them from being fun.
But, as with all things, you need to be careful about overusing the device. I mean, if EVERYONE was a long lost heir, it would lower the impact. Anyway, this was just a random thought I wanted to explore.