Retail, e-commerce, media companies often have a large catalog of products or content, spanning multiple categories. brands, authors, artists etc. By tracking user interaction with these different sections of the catalog, you can learn user preferences, or affinities, for various categories, brands, authors, artists, price-points and more. Savvy marketers use these affinities to tailor marketing messages to every user, on every channel. In the process they end up driving 3-10X higher response rates.