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;
}