Is getString() method is necessary for getting a string resource ?

Yes, within the Activity context, you must use getString(int) or getText(int) to get the string resource.

It only looks like you don't need to do that for TextView.setText() because that class is hiding the call. If you look into the TextView source code, then you can see that there are multiple "overloaded" implementations of setText(). For example, here's a snippet from the TextView source code:

public final void setText(CharSequence text) {
    // Code to set the text here.
}

public final void setText(int resid) {
    setText(getContext().getResources().getText(resid));
}

So, depending on what datatype you pass as the argument to TextView.setText(), a different method will be called. Some classes in Android allow either type of input, some only allow the String. But, either way, the same thing will eventually be called.

I hope this clears it up for you more.

More info: http://developer.android.com/guide/topics/resources/string-resource.html

ps - In the Activity context, getText(int) is just like getString(int), except that it returns a CharSequence instead of a String. So, that allows you to have some extra styling in the string resources file.