Same as anything else, an explicit flag to achieve the same "just give me a port" functionality as this "magic number" does without resulting in the documented unpredictable behavior around a technically legitimate port number.
As the article points out, most if not all of the services exposed on port 0 seem to be accidental and likely resulted from people assuming this magic behavior applies everywhere instead of just to one specific API.
Yeah, obviously I'm a programmer so I'm well aware it's technically possible, it was a bit of an exaggeration to explain how much value I feel I get from it.
> most if not all of the services exposed on port 0 seem to be accidental
No program actually end up "exposed on port 0", if it's not supported it won't be able to create a listener on that port, AFAIK. Most yeah, all no. P2P software commonly leverage port 0, and other "discover<>connect" types of software. But I'd agree that it's most likely accidental if outside of those niches.
Clearly you didn't read the article, plenty of stuff is in fact exposed on actual port 0 because it's a real thing and not just a way to get whatever ephemeral port like you're using.
joker@joker:~$ ssh localhost -D 127.0.0.1:0
Bad dynamic forwarding specification '127.0.0.1:0'
joker@joker:~$ ssh localhost -D 127.0.0.1:9999
ssh: connect to host localhost port 22: Connection refused
joker@joker:~$ ssh localhost -L 127.0.0.1:0:localhost:8080
Bad local forwarding specification '127.0.0.1:0:localhost:8080'
joker@joker:~$ ssh localhost -L 127.0.0.1:9999:localhost:8080
ssh: connect to host localhost port 22: Connection refused