Aliases permit a number of things to happen:
All systems require aliases for Postmaster and MAILER-DAEMON to be RFC-compliant.
Always be extremely aware of security when defining aliases that invoke programs or write to programs since sendmail generally runs setuid-root.
Changes to the aliases file do not take effect until the command
is executed to build the required dbm tables. This can also be done by executing the newaliases command, usually from cron.
Details concerning mail aliases may be found in the aliases(5) manual page.