Answer:
Arguably, fetishes serve an evolutionary purpose. They make it possible for someone who might not meet the conventional standards of beauty to find a sexual partner by adorning themselves with high-heeled shoes, for example.
There's no way of knowing why a particular thing becomes the object of a fetish, but it's interesting to notice that there are a lot of shared fetishes -- feet, for example, or leather. And it's also interesting to note that new fetishes develop. For example, there is a fetish for vinyl, and vinyl is a relatively new material in human history.