No one Postfix programme trusts another programme. For a cracker to attempt to break into a system through Postfix, she would have to fight her way through a half-dozen programs, no mean feat. Another benefit of making each programme separate is that debugging becomes much easier. If mail is received but mail isn't being sent, you know where to start looking. Sendmail, on the other hand, is for the most part one huge programme.
Postfix is also a good citizen.
If it has thousands of messages to be delivered to a remote host which has been down, and it finds that host is back up again, it won't suddenly insist on sending through those thousands of messages. If Postfix notices that the CPU load is too high, or there isn't sufficient disk space, it will hold back. It won't make a bad situation worse. SMTP, the standard Internet protocol for delivering e-mail, is the only mail transport Postfix supports, although there is limited UUCP support. UUCP is rarely used now.