The email mailing lists in Google Groups and Yahoo!Groups have very useful features. If the email lists are set up so that only messages from members are sent to those in the list, then spammers will not be able to spam the list. Such a protection is generally not available in a personal mailing list.
Let us think about this. If your email address is put on the web, or anyone gets it some other way, it is collected and sold to spammers who use it to send you all kinds of messages which you may or may not want. On the other hand, if the message is sent through the mailing list, as long as the message headers on the email do not include individual email messages, then spammers will not be able to send to the members unless they "spoof" a member's email address.
Another advantage of using a central list instead of a list of email addresses, is that some email systems, e.g. hotmail only permit a limited number of addresses in the To: and Cc: list, so if the list is too long, replies to ALL in the list means that some of the people will not get the replies.
The email mailing list counts as a single email address and does not have this problem.
With a centralized list, it is easy to maintain a list with additions of new and deletions of defunct addresses.