The shoes are a sex appeal thing. It starts with high heal shoes. Wearing high heals forces one to walk on their toes, and it raises the back of the leg and ultimately the buttocks. (Ever see a woman bend over with heals, and then without heals? Trust me, there's a difference.) Wearing heals over time will strenghten one's legs, help firm up the buttocks and can acutally help to enhance one's posture as well.
Now normal high heal shoes force all weight down to two points, the heal and the toes. Most platform shoes acutally have more surface area in contact with the ground than the normal high heal shoe thus more area for the pressure to be spread out over.
Now why women go from 4" heals to 5" to 7" or 9", well that's just statement thing, and they are looking to stand out from the crowd.
Now don't get me wrong, I've never understood why women put themselves through this, and don't really think they have to.
When ever I get dances from one of my favorite dancers I always ask her to take off her shoes. I make it sound like she'd doing me a favor, but she is very appreciative to be able to give her feet a rest.
(BTW - If you good at it, and depending on the dancer, it's usually cool to offer a foot rub. Most dancers have tired feet, especially at the end of their shift, and will welcome a little pampering.)