People knowing that you have account yyy@example.com at example.net could use that information in a spear-fishing attack, or know that you're involved in a controversial website, prohibited website, etc.
Emails were not the only things that were stolen. For example, in the Adobe breach, encrypted passwords were stolen. If your email address is shown as being in the Adobe breach, that also means that your encrypted password, password hint, etc. were stolen. For Sony, maybe credit card information.
If this website was only about whether email addresses were leaked, then why would anyone type in their email address into this website (thus leaking your email)?
People knowing that you have account yyy@example.com at example.net could use that information in a spear-fishing attack, or know that you're involved in a controversial website, prohibited website, etc.
Emails were not the only things that were stolen. For example, in the Adobe breach, encrypted passwords were stolen. If your email address is shown as being in the Adobe breach, that also means that your encrypted password, password hint, etc. were stolen. For Sony, maybe credit card information.
If this website was only about whether email addresses were leaked, then why would anyone type in their email address into this website (thus leaking your email)?