The WordPress developers reference page for shortcode_atts() states:

$atts (array) (Required) User defined attributes in shortcode tag.

But I do not understand this definition.

For example, in this piece of the WP Frontend Profile plugin:

$atts = shortcode_atts(
     [
     'role' => '',
     ],
     $atts
);

As far as I understand, shortcode_atts() is used to create a map between the shortcode attributes and a PHP array.

But I don’t see what the purpose of the $atts parameter is.