Why GRANT UPDATE on SEQUENCE in PostgreSQL I have seen postgresql allowing users to Grant update on a sequence. But never seen any update query on the sequence.
Sequence values are usually changed using ALTER SEQUENCE command. I understand the need for GRANT USAGE and GRANT SELECT on sequences. But what exactly does a GRANT UPDATE on a sequence lets you do in PostgreSQL.
The documentation says:

nextval ( regclass ) → bigint
[...]
This function requires USAGE or UPDATE privilege on the sequence.

setval ( regclass, bigint [, boolean ] ) → bigint
[...]
This function requires UPDATE privilege on the sequence.