I'm pretty sure the web is already full of anonymous forums where you don't have to give up your identity by signing in with Facebook.
* It enables you to talk to your FB friends--a group whose opinion that you probably care about & respect--anonymously, so that they don't have to hold back or sugarcoat things.
* It enables us to ensure a reasonably decent quality on the service, by [hopefully] avoiding too many bots or trolls.