I think it's useful when you want to ask something that could be somewhat embarrassing. Imagine someone would "is there anyone with herpes on here", with the intention of learning how other people endure treatment or discuss it with their boyfriend... If anonimity wouldn't exist that question would trigger zero answers. So this is when anonimity is essential. Unfortunately, some people use it as a balaclava to insult others which is really sad :-(
They just don't want you to see who they are. It's okay, it's a feature. If they are just being abusive, you can also report them as Antagonistic, although only if they actually say mean thing that are legitimately mean with the intent of being mean and not constructive at all.