It could be useful to retain a list of a user's previous email addresses in the DB for use in cases of spamming and various other abuses.
To help trace a member who registers and confirms with a valid email address, does something naughty, then changes to an invalid or blank email address making them harder to contact or trace. Also, to compare against new registrations to prevent the same person from re-registering using one of their previous email addresses. More or less the same reason as we retain the IP addresses that a member uses to register or post.What would you want to be able to do with this data? Is it just for reference purposes?