Celebs get paid to wear those dresses. They must fight off designers and then choose whatever and whomever they want... They go for 'go-see's' and 'fittings'.
Then after that, they have differing tax categories: if you can't sit down in it it's just for show and is a tax write-off. If you can sit down in it, it's for work and therefore is taxable, but in a lowish category.
If they're iconic, they get put in museums (the gowns, I mean
xL
sometimes they get given them, (when they're custom made). Often they just take them, don't give them back. If the dress is good the designer gets a few pix in mags and maybe more clients
As for wearing it previously: Google Princess Diana, she did an amazing job recycling outfits for various occasions. I also had a friend who dyed her wedding dress black, ripped the hem off to knee length and wore it to her father's funeral wake which was immediately following her divorce.