^Exposure, big fan base, publicity..
But honestly, I find it wrong whenever celebrities front any brands. Celebrities can't hold the same mystique they did in past times, and more often than not our own knowledge of them usually clash with whatever image they're promoting.
Just look at the Miss Dior Cherie campaign fronted by Natalie Portman. She looks fantastic in it and I like her but it doesn't come close to the campaign fronted by Marina. With a model, it's always them inhabiting the image and the look. With celebrities there's always a 'meet me half way' thing going on.