I've spent some time searching this question, but I have yet to find a satisfying answer. The majority of answers that I have seen state something along the lines of the following:
- "It's just good security practice."
- "You need it if you are running a server."
- "You need it if you don't trust the other devices on the network."
- "You need it if you are not behind a NAT."
- "You need it if you don't trust the software running on your computer."
The only answer that makes any sense to me is #5. #1 leaves a lot to be desired, as it advocates for doing something without thinking about why you're doing it -- it is essentially a non-answer. #2 is strange -- why does it matter? If one is hosting a webserver on port 80, for example, they are going to poke a hole in their router's NAT at port 80 to open that server's port to the public. What difference does it make to then have another firewall that needs to be port forwarded? #3 is a strange one -- what sort of malicious behaviour could even be done to a device with no firewall? If you have no applications listening on any port, then there's nothing to access. #4 feels like an extension of #3 -- only, in this case, it is most likely a larger group that the device is exposed to. #5 is the only one that makes some sense; if you install a program that you do not trust (you don't know how it works), you don't want it to be able to readily communicate with the outside world unless you explicitly grant it permission to do so. Such an unknown program could be the door to get into your device, or a spy on your device's actions.
If anything, a firewall only seems to provide extra precautions against mistakes made by the user, rather than actively preventing bad actors from getting in. People seem to treat it as if it's acting like the front door to a house, but this analogy doesn't make much sense to me -- without a house (a service listening on a port), what good is a door?
Yes, that's what ports do. They split your IP connection into 65,536 separate communication lines, that's the main thing, but that is specifically 65,536 1-on-1 lines, not party lines. When a process on your PC reserves port 80, that's it. It's taken. Short of hacking the kernel itself, it cannot be reassigned or stolen until the bound process frees it.
The SO answer you found it interesting, I was not aware that the Linux kernel had a feature that allowed two or more processes to willingly share a single port. But the answer explains that this is an opt-in parameter that the first binding process has to explicitly allow. And even then, traffic is not duplicated to all listening processes. It sounds like it's more of a "first come first serve" to whichever of the processes are free to read the incoming message at the time it arrives, making it more of a load balancing feature that isn't a useful vector for eavesdropping.