Why is constructor of super class invoked when we declare the object of sub class? (Java)

Because it will ensure that when a constructor is invoked, it can rely on all the fields in its superclass being initialised.

see 3.4.4 in here

Yes. A superclass must be constructed before a derived class could be constructed too, otherwise some fields that should be available in the derived class could be not initialized.

A little note: If you have to explicitly call the super class constructor and pass it some parameters:

baseClassConstructor(){
    super(someParams);
}

then the super constructor must be the first method call into derived constructor. For example this won't compile:

baseClassConstructor(){
     foo(); 
     super(someParams); // compilation error
}

super() is added in each class constructor automatically by compiler.

As we know well that default constructor is provided by compiler automatically but it also adds super() for the first statement.If you are creating your own constructor and you don't have either this() or super() as the first statement, compiler will provide super() as the first statement of the constructor.