You might need to talk to someone in an organization without knowing his real name. Perhaps you want to talk to someone in charge of a service, for example.
The Internet has defined many pseudo-users. Here is a list of the ones you should define on your system (the mail-host server):
Linux systems have several administrative accounts which receive mail. It is a pain to manually log in as one of those users just to check if there is any mail. Instead, you can redirect those accounts' mail to a real user (you?).
Here is a list of those accounts:
On small sites, there may be a single administrator for the complete network. Instead of setting all those aliases to point to him, you may save time and point them to the pseudo-user "admin," which is actually an alias pointing to the administrator.
This may save time.