Whats up with .ck3 files?

Whats the deal with this non-standard file type? Has Paradox ever talked about why they created it? Does anyone know when they made this change to the file system?

Edit: I've been trying to save my edited gamestate by figuring out how to recompress it into a .ck3 file. I don't really know how to replicate the header. Its really a shame I didn't back up the original save. The file starts with SAV and then a unique set of metadata bytes and then the plaintext stuff which, it turns out, is for the main menu and the icon next to the save name, and then it has the rest of the save data compressed with the PK header, which means its in the .zip format. I can replicate all this but the save doesn't properly load due to the incorrect metadata in the header I believe.