I will give my opinion, but it could not be complete. So, in my opinion, they do content, so people will talk about them, talk about their way of dressing, and it will be a kind of "advertising." For example, when Lady Gaga wears one of her dresses (which always were different and "not usual"), all the world will talk about the brand she wears and/or the designer who did that dress.
That's more or less for every celebrity; they wear a certain dress, and people who like them will immediately "advertise" their dress brand or the name of the designer.
Sometimes I think that even the designers work with celebrities to make talk about them both.