Is meta_key used for ordering, filtering, or both?

Column meta_key is used for filtering and partially for ordering. Why partially? It is used for ordering only in combination with meta_value. Columns meta_key and meta_value are connected together like column name and column value in traditional table. For raw SQL select it would be possible to order posts by meta_key but from a practical point of view, do you order your SQL selects by column names?

Let’s assume that we have custom post type event with meta_key _speakers and _attendants. I don’t see point of ordering events by meta_key but I see point of ordering by _speakers value or _attendants value. To do this you have to make such a query.

$query = new WP_Query(array(
    'post_type' => 'event',
    'meta_key' => '_speakers',
    'orderby' => 'meta_value_num',
));

or

$query = new WP_Query(array(
    'post_type' => 'event',
    'meta_key' => '_attendants',
    'orderby' => 'meta_value_num',
));