SAML2HoKAP question

SAML2HoKAP states that (section 2.4):

"The <saml:SubjectConfirmation> element MAY contain a <saml:NameID>
element.  If it does, the latter identifies an attesting entity
different from the subject of the assertion.  If the
<saml:SubjectConfirmation> element does not contain a <saml:NameID>
element, then the attesting entity and the subject are one and the
same."

Why would you do this? 

josh.

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