Automated processes can only do so much to protect forum quality. Moderated posting can be thought of as a temporary transfer of control from automated processes to a human decision maker.
Software can instantly analyze an email to determine whether the sender is allowed to post messages directly to the list or if the message should be sent for moderation. However, the software doesn't recognize the sender as a person who participates in the organization, nor does it have the sophistication needed to determine whether the message contents are on-topic for the mailing list.
Advantages of moderated posting:
Ashish makes the development forum in his project a moderated one as he wants to make sure that all the messages posted to the discussion come to him for approval before they're included in the forum. When a message arrives, he reads the message, and if it's appropriate for the discussion, he accepts it; if not, he rejects it.
Over time, Ashish finds that the traffic in his discussion has increased and he is no longer able to moderate all the posts by himself. So he adds a couple of other senior developers in his project as moderators, who can share the responsibility of moderating the forum.
After a while, Ashish realises that he doesn't have to reject any messages posted to the forum as everyone seems to understand the purpose of the forum and uses appropriate language in emails. So he removes the restriction and make the forum an unmoderated one. Now Ashish and the other moderators no longer receive emails for approval when a user posts a message to the discussion. Messages are directly included in the forum and delivered to the forum subscribers.