Serializable read only deferrable- implications

https://www.postgresql.org/docs/14/sql-set-transaction.html

"The DEFERRABLE transaction property has no effect unless the transaction
is also SERIALIZABLE and READ ONLY. When all three of these properties are
selected for a transaction, the transaction may block when first acquiring
its snapshot, after which it is able to run without the normal overhead of
a SERIALIZABLE transaction and without any risk of contributing to or being
canceled by a serialization failure. This mode is well suited for
long-running reports or backups."

Could anyone expound on the above? What are the real life differences
between the two commands below? Would this be applicable at all to sessions
on a physical replica?

SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE
READ ONLY DEFERRABLE;

SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;

*Michael Lewis | Database Engineer*
*Entrata*