Going back to my foot/shoe fetish studies in college, it's also a power thing.
Those big boots on the guys...they represent power, raw masculinity, roughness, strength and outdoors...at least they used too until everyone other than army and construction workers started wearing them.
High heels represent elegance, glamour and the power of beauty. High heels also change the shape of a woman's body when she's standing, and make many women seem taller than they are in reality. Those impressions continue to be evoked to some degree even if she's not standing on them but her legs are pointing toward heaven. High heels can also effect the shape of the legs when she's on her knees to some extent in that is flexes muscles to elongate the legs.
And yes, it's also about hygiene. Nasty or ill built porns sets (speaking from experience I've had a set flood in the middle of shooting a scene), nails, splinters and saw dust all about.
Finally, it also conveys or evokes a sense of spontaneity to some degree..."he took me so passionately and quickly I didn't even have time to take my shoes off"...as Paris Hilton would say..."That's hot"