Suppose you're exchanging data with a computer on a port <1024, and you know that computer is running some variant of unix. Then you know that the service running on that port is approved by the system administrator: it's running as root, or at least had to be started as root.
And exactly why on earth would you require that?
(as that makes it far more likely that the OS could be comprised by the software - making such software a serious concern for malicious attack)