Second nut used as a locking nut on the first if the first nut does not have a locking provision such as a nylon insert etc etc.
Harder to strip the heads than standard bolts
Maybe I wasn't clear enough - imagine a 16mm bolt (which is an uncommon spanner size) and a 16mm nut on the other end. Or 18mm. Whatever, it's annoying.
Definitely not locking nuts. I know what they are and have never seen them on a modern car. VW now do annoying things like using one time, 'stretch' bolts and other stupid things instead.
I disagree that triple square bolts are harder to strip. The engagement of the spline tool in the bolt doesn't seem as positive, the engagement depth doesn't seem as great and they generally just seem harder to get the 'feel' of - you're never quite as certain that the spline tool has properly engaged and is actually turning the bolt.