i'm still learning about bitcoin, and i learned about the "proof-of reserve"-concept, that online wallets need to accomplish to prove they are having enough bitcoins for all their liabilities.
Now, the only procedure that i know of to do that is the one described in this document: https://www.bitstamp.net/s/documents/Bitstamp_proof_of_reserves_statement.pdf
This document describes how bitstamp is giving a proof of their reserves, by sending their whole cold storage amount to an adress of themself. So far, that makes sense.
What i don't understand is: What purpose has the signing of a selected message by the third party, described at the end of the first page?
Thanks for any help.