If you're not hip to the latest in gastrointestinal research (you aren't?!), one of the biggest scientific breakthroughs in the past couple decades has been the discovery that "good" bacteria actually play a crucial role in overall health, particularly immune and gut health.
These bacteria -- collectively called the human microbiome -- far outnumber your human cells, and disturbing them can result in anything from mild upset stomach to life-threatening infections. So if the appendix holds a reserve of helpful bacteria, it may actually be a kind of second-line immune system organ.
How, pray tell, did researchers go about figuring out what an appendix does, what with surgeons throwing them away all willy-nilly? It's evolution, baby. The team from Midwestern found that different mammal species actually develop appendices independently; in other words, the appendix didn't come from a common ancestor, and once it appears in a species, it sticks around.