Since the dawn of the science of human biology in the modern age, it has been taken for granted that the internal organ known as the "appendix" was vestigial. A structure that is "vestigial" is so-called because it is believed to be a "vestige" of the organism at an earlier stage in its evolutionary biology. Vestigial organs or vestigial body parts no longer have any practical function, but they have not yet disappeared from the organism's biology. Another common example is the human tail bone, which is said to be a vestigial remnant of the days when homo sapiens had tails. Thus, vestigiality goes hand in hand with evolutionary biology. The "useless" appendix has always been explained as an organ left over from the days when the human diet consisted mainly in vegetation; this hypothesis was proposed by Charles Darwin himself and had been a staple in scientific assumptions about the appendix for over a century.
That is, until contemporary researchers began to discern that the appendix actually plays a very important role in human digestion, which has thrown into question the whole traditional assumption that the appendix is vestigial and hence "useless."
I am not a scientist and do not pretend to understand all the biology, but recent research has indicated that the appendix, far from being useless, has a real function in the digestive system (see here and here for two brief articles on the topic). Current findings suggest the appendix's function has to do with the relation between the body and the digestive bacteria within the intestines. That the purpose of the appendix should have eluded science for so long is not surprising, given that it is only recently that the dynamics of the microbial ecosystem within the stomach and intestines have become understood. As the importance of gut bacteria became clearer, researchers realized that the appendix, partially isolated from the digestive system, served as a refuge for helpful bacteria during bouts of intestinal illness. Once the disease cleared, helpful microbes from the appendix could repopulate the gut.
The point is that until science was able to discern the meaning of this organ, it was presumed to be useless, and hence as a vestigial hold-over from a prior point in our evolutionary past. The fact that the actual function of the structure was not known was itself turned into evidence for the evolution of the human body. As we can see, this assumption was not warranted in the case of the appendix, nor should we assume that evolutionary vestigiality is the necessary conclusion when confronted with a biological structure whose function is not readily apparent. What was taken to be evolutionary evidence was nothing more than an organ whose real function was not yet knowable due to the limitations inherent in science until recently.
This is not to say that one must feel compelled to find a mechanical or functional reason for every structure within the human body. I once heard an evangelical Christian radio program where the guest was attempting to assert a practical function for every structure in the human body. This led him to some absurd conclusions about the "usefulness" of nipples on men, which he stated were capable of secreting milk in emergencies.
Not every body part must have a mechanical "function"; in the case of male nipples, these exist because all human beings are born biologically female with the capacity to develop into a male or remain female. This means an embryo must have the structures in place to develop either male or female genitalia depending on the sex of the child. This is the same reason why men have nipples; as an embryo develops, it follows a certain developmental path depending on whether it becomes male or remains female.
The point is there are other reasons our bodies are endowed with certain characteristics other than mere functionality. Just because a structure appears to have no "usefulness" is not an argument that it is a product of evolutionary vestigiality, nor must the Christian feel compelled, on the other hand, to fabricate functions when they are not necessary. We need not assert a practical use to every part of the human organism; nor, however, do we need to conclude that parts of our biology whose function remains unknown are vestigial and implicitly evidence of evolution.