What is the meaning/benefit of this command: export PGOPTIONS="-P"

I see https://www.postgresql.org/docs/current/sql-reindex.html has

$ export PGOPTIONS="-P"
$ psql broken_db
...
broken_db=> REINDEX DATABASE broken_db;
broken_db=> \q

I see the below content, but I still did not understand. > Alternatively, a regular server session can be started with -P > included in its command line options. The method for doing this varies > across clients, but in all libpq-based clients, it is possible to set > the PGOPTIONS environment variable to -P before starting the client. > Note that while this method does not require locking out other > clients, it might still be wise to prevent other users from connecting > to the damaged database until repairs have been completed. What is the meaning/benefit of this command: export PGOPTIONS="-P"