Unix servers generate a lot of e-mail, with most of it being sent to the root user's mail account. When you have more than one or two servers, logging into each and checking mail (generally from the command line) is not effective. Instead, it is better to forward all system e-mail to a centralized account which can then be checked via normal procedures, for all servers set up this way.
The postfix smtp server uses the /etc/aliases file to override the “normal” mail targets. Mail destinations are first read from the aliases file and, only if they can not be matched there, are then sent to individual users on the system.
To send all mail destined for the root user, you must first create an alias or an account on a centralized server, then perform the following to redirect mail to the root user to that account.