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.
I'd be happy with a manual reference (like looking up IP addresses), although it could also be used actively along with IP addresses and perhaps cookies in a "Who Might This User Be" type of modification which automatically flags new registrations which might be "old friends" returning with new names. This latter bit might be better off as an add-on though.