Non-static classes in static classes

Question

What is the rationale for non-static inner classes to be declared in a static class?

public static class A
{
    public class B
    {
        public B() { }
    }
}

Compared to the fact that non-static members and functions cannot be declared in a static class.

Answer 1

What about multiple instances B? Thus, non-static classes nested in static classes do not differ from ordinary non-static classes.

public static class A
{
    public class B
    {
        public B() { }
    }

    private static B b1 = new B();
    private static B b2 = new B();
}

Look at a real-life example of a static class System.Linq.Enumerablethat contains some specific implementations as nested non-static classes.