I am confused about Rvalue References

Consider class like this

class A {
public:
    A() { std::cout << "A()" << std::endl; }
    A(A &&other) {
        std::cout << "A(A &&other)" << std::endl;
    }
    A(A const &other) {
        std::cout << "(A const &other)" << std::endl;
    }
    void operator =(A &&other) {
        std::cout << "operator =(A &&other)" << std::endl;
    }
    void operator =(A other) {
        std::cout << "operator =(A other)" << std::endl;
    }
};

void test(A&& a) {
    std::cout << "test(A&& a)" << std::endl;
}

When creating variable like this

A a;
A&& b = std::move(a);

No constructor is called expect default when a is initialised.

A b = std::move(a);

This way it calls move constructor, as soon as trying to initialise object from rvalue reference.

When we pass rvalue to function

test(std::move(a));

It calls move constructor too. What is the difference between passing rvalue reference to function compared to initialising rvalue reference variable?

And why do we need to create

void test(A&& a);

if

void test(A a);

still calls move constructor? Just to be able to implement different logic?
What essentially rvalue reference function parameter is?