With the number of big players out there [Google, Facebook, Amazon, etc] who have access to our likes, dislikes, shopping habits, among other interests, it's no surprise that our data is out in the market somewhere.
Many times when a user (let's call them User A) purchases an item or searches for something on a site (site A). User A is aware that site A has captured this information but not what might be done with the data.
Here are a few potential scenarios that can happen:
Site A keeps the data for themselves and have their AI generate a list of recommended items User A might be interested in to encourage more puchases.
Only Site A has information about User A.Site A could be working with an advertising network (Ad Network A) and display ads around their website for monetization purposes. Ad network A has a new campaign that wants to target users similar to User A (but only Site A knows this). Site A agrees to serve the ads, and internally targets them to User A. User A sees the ad by Ad network A, clicks on it, and now Ad network A has knowledge of User A and a general idea of their interest.
Now both Site A and Ad network A have information about User A.
Note that the use of an ad network is only an example -- it is not exclusive to them. These are only two possible scenations, but this is to exemplify just how easily 3rd partiesd can obtain information about users without either of the first parties realizing it.
This is why reclaiming data ownership is so important to general consumers. We have a general idea of who should have our information and who shouldn't. If I were to create an account on Site B and purchased a video game (gameB), I know for a fact that I willingly gave idea of who I am and what my interests might be to site B. In the end, it all comes back down to the users / consumers transparently knowing who has their data and what is being done with it. This is not something we want to share with everyone in the world, especially behind our backs. Blockchain has a very good chance of repaving that road of data security for us.