Structures on the body may lose their function to the point that they are more liabilities than functional. The thymus atrophies after birth and ceases to have a useful function after childhood, the appendix is not felt to have a useful purpose in modern civilization (though may prevent infections in primitive cultures) and is more a liability for an adult than a boon (which is why it is usually removed during many laparotomies as an incidental procedure), damaged spleens from trauma probably kill more people than benefit from intact spleens, etc. And some structures are vestigial remnants of larger structures in earlier species, such as eyebrows, which are barely effective anymore in keeping things away from our eyes, and don't serve the communicative function they did before we became verbal. So the presence of a structure does not mean it continues to serve a useful function. Moreover, as our environment, including the bacterial and viral fauna, has changed over the millenia, as structure that once was useful may now increase the risk of serious illness -- like the foreskin.