Without real moderation, a site might wind up publishing rubbish. Once a site is known to publish rubbish, hits plunge. Google slaps down every item published there, even if the item is excellent.
It is a case of a few rubbish items destroying the entire site.
When a user comes to a site and does not see interesting titles on the front page, the user is gone without a single click.
If a viewer sees a title that grabs the eye but the article itself is rubbish; boring, pointless, badly written, virtually copied from Wikipedia, the viewer closes the page.
The viewer does not try another item, assuming one items is rubbish all are rubbish.
The need to actually edit submissions, to ensure that only the best is on the front page, is evident.
Sites which run a spell checker or some other A.I. are very likely to publish low quality items. Sites which simply put the last published on a front page, without reading it, might be advertising the kind of item which chases viewers.