What is wp_check_invalid_utf8?

The following function checks option, if *blog_charset* is not utf8, than return original string. What is the purpose of this?

function sc_check_invalid_utf8( $string, $strip = false ) {
    $string = (string) $string;

    if ( 0 === strlen( $string ) ) {
        return '';
    }

    // Store the site charset as a static to avoid multiple calls to get_option()
    static $is_utf8;
    if ( !isset( $is_utf8 ) ) {
        $is_utf8 = in_array( get_option( 'blog_charset' ), array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ) );
    }
    if ( !$is_utf8 ) {
        return $string;
    }

    ......

    return '';
}