People share a lot of sensitive material on Quora - controversial political views, workplace gossip and compensation, and negative opinions held of companies. Over many years, as they change jobs or change their views, it is important that they can delete or anonymize their previously-written answers.
I know from first-hand experience that Quora writers sometimes decide to go anonymous after they've shared something sensitive. I do this myself from time to time, and I appreciate the option to make that change; this option gives me more comfort in sharing what I know about sensitive topics.