The purpose of an Affidavit of Naturalization after the expiry of an Indian passport is to declare and affirm the individual's acquisition of a foreign nationality and the subsequent loss of Indian citizenship. This affidavit is typically required by the Indian government or authorities when an individual applies for a new Indian passport after obtaining citizenship of another country. By signing this affidavit, the individual acknowledges that they have voluntarily renounced their Indian citizenship and provides legal documentation to support their claim.