“long thought vestigial”
The other end of that same stick is we still have appendices (well, maybe you do, I don't) because we still need them. If they were truly vestigial they would eventually go away especially since they do case often lethal problems.
For example, left untreated my appendicitis would have prevented me from passing on my genes. There must be some offsetting advantage to compensate for the losses caused by appendices, or the occasional freak born without one would take over the gene pool.
Die of appendicitis? Die of the after effects of diarrhea? Multiple bouts of diarrhea remains a very popular way for small children to die world wide, even with an appendix to repopulate the gut after the first mild case.
Like Perfect Creation, Evolution predicts that everything has a function. We may occasionally discover the function where it isn't glaringly obvious. This looks like one of those cases.