Suppose you had an OnlineIdentity which contains a profile of you. Say even GovernmentBackedAuthentication. However, sometimes you want an AnonymousIdentity.
A ShadowIdentity is the answer. It provides a (possibly improper) subset of your profile (too much information may "fingerprint" a person) without a name or legal address or any personally identifying information. Thus, you can do things like claim you are over 21 without identifying who you are.