Ecommerce sites use your personal info to charge you more. From the article on the EFF site:

CNN has a report detailing an unsavory practice euphemistically called “price customization.” In short, websites examine the web data you automatically shed (cookies, IP address, etc.) so they can charge you a different price for a product or service depending on your “identity” and shopping habits.

The article cites “a retail photography Web site charging different prices for the same digital cameras and related equipment depending on whether shoppers had previously visited popular price-comparison sites” and “one [Amazon] buyer [who] deleted the electronic tags on his computer that identified him as a regular customer and noticed the price of a DVD changed from $26.24 to $22.74.”