Before 4.8, if you register the meta boxes using the old way (with admin_init hook), you could loop through the ['fields'] keys of the global variable $meta_boxes to get all fields registered for a meta box. Here is an example of the old way:

global $meta_boxes;
if ( isset( $meta_boxes['meta_box_id']['fields'] ) ) {
    foreach ( $meta_boxes['meta_box_id']['fields'] ) as $key ) {
        $value = rwmb_meta( $key['id'] );
        if ( ! empty($value) ) {
            echo '<label>' . $key['name'] . '</label> ' . $value;
        }
    }
}

This loop was very handy if your meta box has, say, 20 fields.

Since 4.8, the $meta_boxes is not used as a global variable anymore. You’re recommended to use rwmb_meta_boxes to register meta boxes and fields. But it’s still pretty easy to loop through the Meta-Box-only field keys using RWMB_Core::get_meta_boxes() function. The code is almost identical:

$meta_boxes = RWMB_Core::get_meta_boxes();
if ( isset( $meta_boxes['meta_box_id']['fields'] ) ) {
    foreach ( $meta_boxes['meta_box_id']['fields'] ) as $key ) {
        $value = rwmb_meta( $key['id'] );
        if ( ! empty($value) ) {
            echo '<label>' . $key['name'] . '</label> ' . $value;
        }
    }
}

Note: Without this function you would be trying to loop through ALL post meta, and there may be WordPress meta fields that are not part of your Meta Box, and you don’t want those. Without the core function, to retrieve only the values for your meta box with get_post_meta, you would have to know and re-list all the field keys separately in your code.