Answer 1

They are very important for scenarios where polymorphism is used. Consider the following classes:

class A {
public void print () {
System.out.println("A");
}
}

class B extends A {
public void print() {
// we're overriding the behaviour from A
System.out.println("B");
}
}

Now consider creating two objects:
A objA= new A();
A objB= new B();

Since the behaviour is different for the concrete objects, although stored in the same type of A, calling print on both objects will print different results on the console output:

objA.print(); // prints A
objB.print(); // prints B