The appendix is not useless — it stores beneficial gut bacteria

For over a century, the appendix was considered a vestigial organ with no function, notable only for its capacity to become lethally infected. A 2007 paper by Bill Parker at Duke University proposed a different function: the appendix acts as a 'safe house' for beneficial gut bacteria. After a severe intestinal infection or diarrhoeal illness that wipes out the gut's bacterial population, bacteria from the appendix help recolonise the colon. Populations without appendixes (due to removal) have slightly higher rates of Clostridioides difficile infection after antibiotic use — consistent with the safe-house hypothesis.

Source: Bollinger et al., Journal of Theoretical Biology (2007)