This would allow the case where an anonymous user needs to be able to query a table for more information on a specific row, without having access to every row that they do not know about.

For example, if an anonymous user needs to access the primary key attached to their email address (perhaps to fetch other non-sensitive information connected in another table via key_relation to the primary key), they would pass their email address in the x-hasura-user-id header so that they can query-select their specific row without being able to query-select every row (including other users' emails) in the entire table.