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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