How valuable is data for advertising? From the article:
If a company that sells online ads can know what their readers are reading on other sites, then they can target the users based on that information when the user returns to their own site. For example, say Michael visits CNBC’s website in the mornings and reads about the markets, but visits The New York Times in the evenings and only reads the book review section. CNBC knows Michael is someone who follows the markets, and might monetize his view at a $30 CPM. The Times knows that Michael is someone who likes to read books so might only monetize Michael at a $10 CPM. If the Times can somehow find out that Michael is reading CNBC in the mornings, then when Michael visits the Times book section in the evening, the Times can target him as someone who follows the markets and monetize him at $30, too.