Running nested CTEs - sql

Question

On the BigQuery SELECT syntax page it gives the following:
query_statement:
query_expr
query_expr:
[ WITH cte[, ...] ]
{ select | ( query_expr ) | set_operation }
[ ORDER BY expression [{ ASC | DESC }] [, ...] ]
[ LIMIT count [ OFFSET skip_rows ] ]
I understand how the (second line) select could be either:
{ select | set_operation }
But what is the ( query_expr ) in the middle for? For example, if it can refer to itself, wouldn't it make the possibility to construct a lisp-like query such as:
with x as (select 1 a)
(with y as (select 2 b)
(with z as (select 3 c)
select * from x, y, z))
Actually, I just tested it and the answer is yes. If so, what would be an actual use case of the above construction where you can use ( query_expr ) ?
And, is there ever a case where using a nested CTE can do something that multiple CTEs cannot? (For example, the current answer is just a verbose way or writing what would more properly be written with a single WITH expression with multiple CTEs)

Answer 1

The following construction might be useful to model how an ETL flow might work and to encapsulate certain 'steps' that you don't want the outer query to have access to. But this is quite a stretch...
WITH output AS (
WITH step_csv_query AS (
SELECT * FROM 'Sales.CSV'),
step_filter_csv AS (
SELECT * FROM step_csv_query WHERE Country='US'),
step_mysql_query AS (
SELECT * FROM MySQL1 LEFT OUTER JOIN MySQL2...),
step_join_queries AS (
SELECT * FROM step_filter_csv INNER JOIN step_mysql_query USING (id)
) SELECT * FROM step_join_queries -- output is last step
) SELECT * FROM output -- or whatever we want to do with the output...

Answer 2

This query might be useful in that case where a CTE is being referred to by subsequent CTEs.
For instance, you can use this if you want to join two tables, use expressions and query the resulting table.
with x as (select '2' as id, 'sample' as name )
(with y as ( select '2' as number, 'customer' as type)
(with z as ( select CONCAT('C00',id), name, type from x inner join y on x.id=y.number)
Select * from z))
The above query gives the following output :
Though there are other ways to achieve the same, the above method would be much easier for debugging.
Following article can be referred for further information on the use cases.
In nested CTEs, the same CTE alias can be reused which is not possible in case of multiple CTEs. For example, in the following query the inner CTE will override the outer CTEs with the same alias :
with x as (select '1')
(with x as (select '2' as id, 'sample' as name )
(with x as ( select '2' as number, 'customer' as type)
select * from x))