Consider a biological structure which no longer benefits an organism, such as the eyes of an organism whose population now lives in total darkness. I can think of three reasons why such a structure might disappear:
0) Random changes to the structure over time wouldn't be corrected by selection favoring the functional version of the structure, leading to a wider variation where most versions of the structure no longer effectively function.
1) The resources the structure demands could be better spent on structures which are actually being used; e.g. human eyes require a lot of blood that could be used elsewhere.
2) Perhaps the existence of a very complex structure leads to biological problems which would no longer be an issue if the structure were not present; e.g. human breasts plus breast hormones frequently leads to cancer.
Are these three examples reasonable means by which a feature would disappear? Are there any other possible reasons?
Is there a general name for the phenomenon of evolutionary removal of vestigial features due to those features no longer being useful to a population?