Normal mailing list allows all members to post to the list and broadcast the message to every other member in the list. This is great for discussion list, but definitely not what you want for an announcement only list.
You can create an "announcement newsletter" that allows only a few select people to send a message to all the list subscribers, but prevent normal subscribers from doing the same. This can be achieved by creating a mailing list with the following options:
Posts from addresses other than moderators are rejected. This is applicable to message moderated lists only