What are the benefits of polymorphism?

For example, if class B extends A, we can initialize it by A a = new B(); we can also initialize it by B b = new B();

in Java, I usually use List list = new Arraylist rather than ArrayList list = new Arraylist, but I do now know what the advantages are.

I find some answers in google, someone says, we can create a list to add a and b if use A a = new B

but I think it also works if we create a list of A and add object b since b is a subtype of A.