Why we have organs that we do not use

I have always found it fascinating that we have organs that we do not need as humans. We all know about the appendix, gall bladder and so forth. My question to Ask A Scientist was about why males have mammary glands when they do not produce milk and do not use this body part for anything. I have not heard an answer to this question. I am under the impression that it does take a while if ever that an answer is given.