For a variety of reasons expats can need to attest to their identity for the purposes of some administrative or legal process back in their home country.
Popular reasons include: the sale or transfer of a property in the UK requiring authorisation from the owner who is living abroad. A lawyer is often required to verify the sellers identity so that the sale can proceed.
Another common reason would be to validate a person's identity for the purpose of applying for a replacement passport or indeed for the relase of pension funds.
In the UK Sometimes such services are provided by a Commissioner of Oaths.