> Transient data set.

AFAIU the key differences from standarts
CREATED LOCAL TEMPORARY TABLE + INSERT INTO is

1) the creation moment, which for "transient data set" is DML
2) ability to use it in PSQL

right ?

If so - where the benefit of "transient data set" before the
standart WITH ?

Transient data set can't be indexed, therefore joins with it can
be slow.

Regards,
Vlad

PS CREATED LOCAL TEMPORARY TABLE can be easy build on top
of GLOBAL TEMPORARY TABLE, imho