What function does the appendix have or had?

The contains reserves of good bacteria (the intestinal flora) that the needs to function. This is in diseases with severe diarrhoea, such as cholera; the affected recover faster thanks to reserves.

In addition, the appendix is part of the lymphatic system and therefore plays a role in the immune system. It is also believed to “water” some white cells that they can recognize and prepare for attackers.