Enter a question and answer only you are likely to know so in the case of you losing your login details or perhaps changing your email address we can ask you the secret question to which you can provide the correct answer to, thereby confirming your identity to us.
An example might be;

Q. What was my first pet's name?
A. Fluffy

Your Email address is critical for us to be able to identify you and communicate to you.

You use your email address and password to login to the system.