Why do mailing lists have two or more addresses associated with them, and what are the addresses used for?

Answer 1

When you sign up for a mailing list, the automated system usually emails you back with instructions on how to use it.
Different mailing lists use different software, so its not going to be the same for every one.
Some systems, using different email addresses, allow for:
Searching and returning items with keywords you supply.
Retrieval of history items or groups of items based on the order they were received.
A list of currently subscribed users.
Switching between normal and digest format.
Vacation mode.
etc.