xtrabackup prepare - after dump or before restore?

Question

i have doubts about xtrabackup --apply-log. is there any difference when use it? should it be done right after dump or before restore (even few months later)? seems stupid, but maybe i missing something.

documentation says this:

Prepare a backup in BACKUP-DIR by applying the transaction log file named xtrabackup_logfile located in the same directory. Also, create new transaction logs. The InnoDB configuration is read from the file backup-my.cnf created by innobackupex when the backup was made.

thanks for help :)