It comes down to what people's goals are: people run a forum, a discussion platform, because they want to see a community thrive. They want to see people interact. They want to be part of that interaction. Either through social means, or special interest, or aligned goals. They want a community.

Moderation fits into those goals, because it keeps the conversation relevant to the community that is being grown. Moderators are curators of a garden.

The internet is filled with totally unmoderated content, that would be your random blogger website that Google also indexes. The reason there's value in the communities is because of the moderation, because of the focus, because of being kept on topic, because of the spam removal. That's why Google sees those results as more relevant. That's why you want to be there.

Usenet and blogs exist right now, without moderation.. you could use them