The appendix is located in the lower right abdomen and it's just kind of, well, there. WebMD breaks it down like so:
"The function of the appendix is unknown. One theory is that the appendix acts as a storehouse for good bacteria, 'rebooting' the digestive system after diarrheal illnesses. Other experts believe the appendix is just a useless remnant from our evolutionary past. Surgical removal of the appendix causes no observable health problems."
So, unless your appendix gets inflamed and has to be surgically removed, it doesn't seem to really do a whole lot. But useless (and in fact potentially harmful) biological organs usually don't last long thanks to evolution, which suggests that it must be good for something. And there's new research to back this up, showing that some form of an appendix has independently evolved 32 separate times among mammals.
The answer could be that it actually does have a function: to keep our good digestive bacteria protected when a major infection threatens our health. That doesn't mean that everyone who's had appendicitis is in big trouble, but it does mean that we might not have all the facts about the appendix. Similar structures have been found in beavers, koalas and porcupines — and a lot of other diverse species. It's a lot more common than previously thought, making it likely that the organ does more good than we're giving it credit for.
It sounds like there's still a lot we don't know about this little anatomical enigma, but scientists are in hot pursuit of answers.