Other people's fetishes that we don't share are rarely understandable. However, in the porn industry, the foot fetish is a major one, huge audience - higher than BDSM by a considerable margin, although it is fragmented. Within the overall foot fetish stuff there are lots of divisions and sub-genres, several of which are mutually exclusive. For some it's the toes, for others the soles, for some it's smelly or dirty feet, while for others it isn't the actual feet themselves at all, but rather the socks or stockings or shoes...
I think the movie Pulp Fiction had a great line about it, in that the feet are a surprisingly intimate body part. You don't just let anyone touch your feet. In psychology, a person who doesn't take their socks off to have sex is considered a common sign of lack of commitment - they've maintained a certain readiness to walk/run away.
Ultimately, isolating any one body part from the overall experience is kind of fetishistic and equally odd. I mean, is an obsession with the foot really so different to an obsession with a sack of fatty tissue that is meant to feed offspring? If you take a dispassionate and scientific outlook on things, most tastes and preferences are pretty crazy. I mean, why all the fuss for redheads? What's so special about being genetically less-suited for global warming?
