Each coin is registered to an email address and password.  You need access that that info to redeem the coin, so its serves as a second form of authentication.  You can change the registration info at any time if needed.  For example, if the coin changes hands privately, the new owner will want to register the coin with an email address and password that only he or she has access to.

It's protection against the hacking and counterfeiting that has been done to Casascius coins.