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.
So the idea that your appendix is obsolete doesn't make much sense if all these species are developing one independently; the fact that it never disappears once it shows up in a species suggests that, yeah, it's doing something important in there.