Evolution is a funny old thing. On the one hand, it gives us opposable thumbs (or should that be on both hands?), but on the other hand, it ensures that we spend at least half of our lives with crippling lower back pain. The lord giveth and the lord taketh away.
The problem is that, in the words of infamously angry science-ent, Richard Dawkins, evolution is a "blind watchmaker". It can't see what it is doing, but uses trial and error to put something together that appears to work flawlessly, even if it got there by accident.
Sometimes this results in humans walking on two legs, thereby freeing up their hands to carry tools, thereby becoming prolific hunters, but the trial and error approach tends to leave its mark on the finished product. The human body is rife with evidence for our haphazard, messy, convoluted evolutionary past and these vestigial traits are the result of various trade offs, compromises and side effects of the evolutionary journey.
Whilst some are relatively harmless, such as the tailbone that was once, get this, our tail, others have resulted in some less than ideal outcomes. These range from your proclivity to occasionally inhale your favourite snacks, to even your most depraved sexual fantasies.