A Boing Boing reader says: "Immunologists from Duke University believe they've found the function of the supposedly useless and often dangerous appendix: It's a reserve store of good germs to 'reboot' your digestive system in case another bug wipes out the germs necessary for human survival."
The function of the appendix seems related to the massive amount of bacteria populating the human digestive system, according to the study in the Journal of Theoretical Biology. There are more bacteria than human cells in the typical body. Most are good and help digest food.Link
But sometimes the flora of bacteria in the intestines die or are purged. Diseases such as cholera or amoebic dysentery would clear the gut of useful bacteria. The appendix's job is to reboot the digestive system in that case.