Why Static Classes Are Allowed In Interfaces

I am reading about interface and i see that classes are allowed inside interfaces which are implicitly static. Here is sample of code i created and i am able to access the static method and fields as well. Here is the code snippet.

public class TestInnerClass {
public static void main(String[] args){
Test.NestedClass.printMe();
}
}
interface Test{
static class NestedClass{
static int x = 100 ;
public static void printMe(){
System.out.println(x);
}
}
}

My question is what is the use of such static classes inside interface? If i don't have access to Foo, i can't ever invoke NestedClass. Whats the design usage?