Por que, se você não especificar --backup-dir, a opção --omit-dir-times estará implícita?

Question

A partir do manpage do rsync

-b, --backup

With this option, preexisting destination files are renamed as each file is transferred or deleted. You can control where the backup file goes and what (if any) suffix gets appended using the --backup- dir and --suffix options.

Note that if you don't specify --backup-dir, (1) the --omit-dir-times option will be implied.

Qual é o motivo pelo qual

if you don't specify --backup-dir, (1) the --omit-dir-times option will be implied?

Obrigado.