That is, say you have a relatively complex backend mutation that needs to schedule some async work (eg sending an email after signup). With a Postgres queue, if you insert the job to send the email and then in a later part of the transaction, something fails and the transaction rollbacks, the email is never queued to be sent.
The big advantage is for code paths which async modify your DB; these can be done fully transactionally with exactly-once semantics since the Job consumption and DB update are in the same transaction.