They occur in animals (and plants) which have changed their style of life from their ancestors. Thus snakes lost their legs as their system of movement changed. But one type of snake – the boas – have vestigial rear legs and pelvis. The human vermiform appendix is another example. That was much larger, and stored microbes which produced cellulase to break down plant cell walls. Leaves are the main diet of apes, but they are not a main part of man's diet. Cellulose cannot be digested by our species.