One of my favorite books, Idlewilde, is about a simulation that we created for just 9 people because the rest died of an incurable disease, and these 9 would need to repopulate the human race. A very good reason to have a simulation in my opinion. The book handles the wakeup in a pretty interesting way.

The best part is nested simulations -- if you're toying around with simulations like Oculus Rift and then you wake up from Earth-sim, it starts to become a game of "where the actual f_k am I really?"