Anonymous structure

[Tofu Ninja via Digitalmars-d-learn]

Just out of curiosity, what is the point of the following?
struct a{
        struct{
                int x;
                int y;
                int z;
        }
}

As far as I can tell, the anonymous structure does nothing. How 
is it different from

struct a{

        int x;
        int y;
        int z;
}

Also is there a way to have a named substructure, not a nested 
structure but something to just add an additional name, maybe 
something like
struct a{
        struct{
                int x;
                int y;
                int z;
        } b;
}