The appendix is a weird thing. Its full name is vermiform appendix, and it's a vestigial organ at the base of the large intestine, packed with useful bacteria once used to help us break down high-cellulose veggies. Charles Darwin compared vestigial organs to silent letters in words; the letters aren't pronounced, but they offer clues to a word's origins. Over the ages, as our diet became more refined, that original purpose of the appendix was lost, and we were left with a worm-like thing hanging out in our guts.
However, recent research suggests the vermiform appendix may have actually evolved into serving some other purpose in conjunction with the immune system; when good bacteria is wiped out of the digestive system because of some illness, that useful bacteria can be replaced by the stuff sitting in the appendix. This is especially useful in under or undeveloped areas, where sanitation is lacking (like the tropics).
In other words, the appendix is a kind of security system. “Vermiform” also means “shape of a worm.” Seen any worm-shaped security systems lately?