This article is three years old, but the notion that the appendix is totally useless has somehow persisted, at least for me.
“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.The appendix “acts as a good safe house for bacteria,” said Duke surgery professor Bill Parker, a study co-author. The location of the appendix — just below the normal one-way flow of food and germs in the large intestine in a sort of gut cul-de-sac — helps support the theory, he said.
The article also discusses the fact that the appendix may have lost its usefulness for people living in populations where diseases like cholera have been eradicated. Hence the modern idea of the useless appendix (and the higher rate of appendicitis in developed countries).