Reputation becomes important in pseudo-anonymous and de-anonymised sites. This often leads to popular individuals being able to take control of a discussion. Anonymous posting prevents one or two individuals from controlling discussion and acceptable viewpoints by sheer force of their reputations.
Posters may have ulterior motives for suggesting a topic or controversial viewpoint that does not add to the discussion. An example of this is marketers manufacturing angry discussions over their products to lure in regular users and spread awareness of the product.