Ports below 1024

As far as I know this is, indeed, mainly just an historical convention; the idea being that when accessing a port under 1024 you can be sure you're accessing whatever the administrator of the server configured to run on the server. This made more sense back when servers where few and huge and you needed an easy way to authenticate, or at least judge the reliability of a service, by such basic means.

By the way, you may find that Capabilities do what you want. See this SO question for more information on the alternatives, but here's the sample use:

setcap 'cap_net_bind_service=+ep' /path/to/program