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Binding to ports under 1024 traditionally requires root privileges. (These days of course that isn't quite as true as it used to be.)

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It's common practice to open the socket to start listening on the <1024 port, then drop the root privileges and continue as a different user.

On a modern Linux system, it’s better to use the CAP_NET_BIND_SERVICE capability instead. Then you don’t need to start the process as root at all.

Why on Earth do ordinary systems need CUPS binding to a port at all?

They don't need. Want, for listening to network printers announcing themselves. It's a very bad system, even then.

Have you ever seen a network printer announce itself by talking to a print daemon on port 631?

I’ve seen network printers announce themselves over DNS-SD/mDNS and over NetBIOS, AppleTalk, etc. All of those are a layer beneath the print daemon.

They don't. cups-browsed is a legacy component that isn't needed on ordinary systems, which outsource printer discovery to an mDNS service such as avahi.

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