Variable_get question default value in form

So I am reading Drupal Development Pro 7 and have stumbled upon something I am a little bit unsure about to do with the variable_get function & form element. I understand it retrieves a stored value from the database and the second parameter is a default value if it cannot find anything in the database matching that form element.

My question is what does the second parameter do in this example below.

foreach($types as $node_type){
    $options[$node_type->type] = $node_type->name; //access the $node_type object and store specific properties from the node_type object in $options array
}

//Checkboxes on Form
$form['annotate_node_types'] = array(
    '#type' => 'checkboxes',
    '#title' => t('Users may annotate these content types'),
    '#options' => $options,
    '#default_value' => variable_get('annotate_node_types', array('page')), //when using the system_settings_form, the name of the form element must match the key of the variable_get ie $form[] = variable_get('$form[]', '') Always form_element name
    '#description' => t('A text field will be available on these content types to make user-specific notes.'),
);