LEFT JOIN used in psql describe.c

Does anyone know why so many LEFT JOINs are used in psql/describe.c to
join to the pg_namespace table, like here:

printfPQExpBuffer(&buf,
"SELECT c.oid,\n"
" n.nspname,\n"
" c.relname\n"
"FROM pg_catalog.pg_class c\n"
" LEFT JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace\n");

processNamePattern(&buf, pattern, false, false,
"n.nspname", "c.relname", NULL,
"pg_catalog.pg_table_is_visible(c.oid)");

I thought a pg_class row always pointed to a valid pg_namespace row because
of our dependency restrictions.

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