Why interface is so important since we can still perform the same function of a class without implementing it?

interface Humanoid {
    // here's a default method to return a name for this humanoid.
    public String getName();

    // get the humanoid's health.
    public int getHealth();

    // set the humanoid's health.
    public void setHealth(int amount);
}

class Zombie implements Humanoid {
    String name; // class level name variable
    int health; // class level health variable

    // constructor method (for creating a new Zombie)
    Zombie (String humanoidName, int healthAmount) {
        name = humanoidName;
        health = healthAmount;
    }

    // implementing the "getName()" method from the Humanoid interface. Now we must fill out the details.
    public String getName() {
        return name;
    }

    public int getHealth() {
        return health;
    }

    public void setHealth(int amount) {
        health = amount;
    }
}

class Skeleton implements Humanoid {
    String name;
    int health;

    Skeleton (String humanoidName, int healthAmount) {
        name = humanoidName;
        health = healthAmount;
    }

    public String getName() {
        return name;
    }

    public int getHealth() {
        return health;
    }

    public void setHealth(int amount) {
        health = amount;
    }
}

class Main {
    public static void main() {
        //lets start off by making a zombie and a skeleton
        Zombie zombie1 = new Zombie("Bob Zombie", 20);
        Skeleton skeleton1 = new Skeleton("Skelly", 10);

        // lets use the method below (isZombie) to tell whether or not we have a zombie
        isZombie(zombie1); // output would be "true"
        isZombie(skeleton1); // output would be "false"
        // see, you can use either class type in this method without having to create two separate methods, saves you time and code length

        // we can also do damage to these entities using the "hit" method below
        zombie1.getHealth(); // health = 20
        hit(zombie1);
        zombie1.getHealth(); // health = 18

        // now lets create another Zombie and another Skeleton, but instead of using their actual class types, we'll allow for manipulation by using their interfaces
        Humanoid humanoid1 = new Zombie("Brains", 12);
        Humanoid humanoid2 = new Skeleton("Bones", 5);
    }

    // if you want a method that could take any class that implements your interface, you can use the interface as a variable
    public boolean isZombie(Humanoid humanoid) {
        if (humanoid.instanceOf(Zombie))
            return true; // returns true if the inputted class is a Zombie class
        return false; // returns false if the inputted class is NOT a Zombie class (for example, a Skeleton class)
    }

    public boolean hit(Humanoid humanoid) {
        humanoid.setHealth(humanoid.getHealth() - 2);
    } 
}