After reading the other links I still don't get it, can you give an example of these external systems?
An external system such as Gridcoin, SPARC, or any other cryptocurrency which decides to reward BOINC computation.
And what's the whole purpose of it?
To prove BOINC account ownership, so that we can securely distribute rewards.
Is it to ease down the load on those project servers and to be more or less anonymous or is/was there a security risk involved?
You wouldn't need to contact a project to verify the ownership of an UserID because you could verify that the project signed the data using their public key. It would make it more anonymous because you wouldn't need to provide the gridcoin client an email address anymore, and yes there are potential security problems right now within the gridcoin network which could be neutralized if we were to use this proof of userid ownership mechanism.
Got it. Thanks for the detailed explanation :)