Most people use the same e-mail for every single account they have. A large majority of these users use the same password for all of their accounts. (Just want to clarify that I do neither of these things - I have a large set of e-mail aliases and have a unique & secure password for each account I have to set up manually).

If you'll grant me that fact, then all I need is your e-mail from a dump of AnyList's users table, and look up that e-mail in my already vast database of dumped tables, and see that your password was "hunter2". Now I have access to your bank account, because you used the same e-mail and password for that account as well.

This is a bit of a contrived example, but in general, any personal information that is leaked (e-mail included) is bad - full name, address, and the like, which many websites ask for, is even worse, because crackers have even a better shot at guessing a lot of your personal information, and at that point, the ball is in their park.