Easy, to make money, by selling your merchandise, which utimately contributes to the advertisement of your site/business.
I personally like Cafepress, maybe just because it's a gigantic outlier in the shop business compared to some other related services, but it's probably not the most personalized of shops out there. Still, if you're a business who doesn't want to deal with the coding of your own shop, Cafepress is a great solution.