What is the first parameter passed to wp_add_dashboard_widget() callback?

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This may be obvious to some, I can’t figure it out.

The wp_add_dashboard_widget function allows us to add a new Dashboard widget like so:

wp_add_dashboard_widget(
  'my-widget',//id
  'My Widget',//title
  'callback_to_echo_widget_html',//callback
  null,//in my case this set to null as I do not use this callback
  array( 'additional_callback_param' )//$args property of the widget 
);

Now, we can use our callback like so:
function callback_to_echo_widget_html( $ominous_first_param, $callback_args ){}

What I cannot figure out is why this mysterious first param in above callback?
What is it for? What should it do? Why is it empty?

Probably I miss something obvious, however I couldn’t figure this one out 🙁