My job while I was in college entailed calling people to query them about their grocery shopping. We had to rattle off long lists to see if people had purchased certain products that day. Potatoes, Other Vegetables, Dog Food, Cat Food, Pasta, Chips, Soda, etc.
Each completed survey netted the company a pretty penny (glad I got paid by the hour, though). Needless to say, this self-reporting was somewhat inaccurate.
Then came loyalty cards. For a fraction of the cost companies were suddenly able to very accurately track their customers' purchasing habits. I estimate that the loyalty cards enable companies to obtain very detailed, accurate data on at least 50-100 customers for the same cost of a single interview back then...
It's not merely your name/address you're selling for a few pennies to each merchant, but your entire, detailed shopping life.