twentysixteen_content_width apply_filters

function twentysixteen_content_width() {
$GLOBALS[‘content_width’] = apply_filters( ‘twentysixteen_content_width’, 840 );
}
add_action( ‘after_setup_theme’, ‘twentysixteen_content_width’, 0 );

In the above codes, why not use:
$GLOBALS[‘content_width’] = 840;