Privacy is a limit on the power of the private sector and the way it can influence us. This data isn't just used to sell in a straight forward manner. Advertisements following a person around the Internet are just the tip of the iceberg.
Personal and meta data can, and is, used to target individuals in both the public and economic spheres and not only in good ways. In a congressional meeting in 2013 the World Privacy Forum revealed lists for sale such as; rape sufferers, erectile dysfunction sufferers, alcoholism sufferers and AIDS/HIV sufferers. There were a further 500 variations on this theme at the cost of $79/1000 people. To quote Forbes,
The text describing each list is the same (but you swap in the affliction of choice for "rape"): "These rape sufferers are family members who have reported, or have been identified as individuals affected by specific illnesses, conditions or ailments relating to rape. Medbase200 is the owner of this list. Select from families affected by over 500 different ailments, or who are consumers of over 200 different Rx medications. Lists can be further selected on the basis of lifestyle, ethnicity, geo, gender, and much more. Inquire today for more information."
To give an idea of the size of the data brokerage industry and the power it can exert over peoples lives,