What is the advantage by keep reference from superclass and point it into the subclass in dynamic polymorphism?

Question

I'm new to the java and I have a question about dynamic polymorphism. I created a class as follows. This is my superclass.

class Student{
    protected String stuName;

    Student(String stuName){
        this.stuName = stuName;
    }

    public String getStuName() {
        return stuName;
    }

    public void study(){
        System.out.println("undergraduate");
    }
}

I extend this class as follows, to take two subclasses.

class ScienceStudent extends Student{
    ScienceStudent(String student){
        super(student);
    }

    @Override
    public void study() {
        System.out.println("science undergraduate");
    }
}

class ComputerStudent extends Student{

    ComputerStudent(String stuName) {
        super(stuName);
    }

    @Override
    public void study() {
        System.out.println("computer undergraduate");
    }
}

Then after this is my main class.

public abstract class Main {
    public static void main(String[] args) {
        ScienceStudent scienceStudent = new ScienceStudent("jerry");
        showName(scienceStudent);
        Student student = new ScienceStudent("tom");
        showName(student);

        ComputerStudent computerStudent = new ComputerStudent("albert");
        showName(computerStudent);
        Student student1 = new ComputerStudent("lee");
        showName(student1);
    }

    public static void showName(Student student){
        System.out.println("Student :"+student.getStuName());
    }
}

I created a method showName to show the name of the students. I used its parameter type as Student(the superclass type) Then after I instantiate the object as above. I want to know that is there any advantage by making object reference of the superclass and point it into the subclass. That's mean what is the difference of

ScienceStudent scienceStudent = new ScienceStudent("jerry");

and

Student student = new ScienceStudent("tom");

I used both of two implementations, but both of them show same result for the showName method as follow.

Student :jerry
Student :tom
Student :albert
Student :lee

So what is the advantages of this one?

Student student = new ScienceStudent("tom");