Why do you request an e-mail address in order for me to establish a member profile?

A person's e-mail address is used as a means of authenticating their identity, and as a search term in our database.
E-mail is the identifier for the set up of an on-line profile. If there is an attempt to use an existing e-mail address this message appears. 
Warning: [e-mail] is already in use by anther user. You must enter a unique email address to register. Please try Again.