How data is linked to you
The reason data is valuable is because it can supplement a lot of information from a little. Data online gets organized and is accessed across sites by unique identifiers, or "keys". The most common being an email address.
Whenever you use the same email in multiple places, all information that has been linked to that email in the past is now accessible to whomever has the email now.
Example: If you sign up to a site with the same email you used to sign up for facebook, the new site can now purchase all of your personal data from facebook that they have collected that has been linked to the same email.
It's not just facebook. In fact there's entire companies whose business model is to aggregate as much data from as many sources as possible, and then sell the complete picture. You probably haven't heard of many of them. We'll talk about them in the next section.
You may think you are just giving them an email, but you are giving them your entire identity.
Not Just Emails
This applies to any identifier that is the same across multiple sites. Name, address, phone number, payment info, the list goes on and on.
We are designing tools to keep you covered on any possible identity leak point.