You most likely don’t need on device firewall if your in your home network behind a router that has a firewall. If you‘d disable that firewall as well and one of your devices has e.g. SSH activated using username and password, than there is nothing stopping a "hacker" or "script kiddy" from penetrating/spamming your SSH port and brute force your password. The person than can take over your PC and can e.g. install software for his botnet or install keylogger or can overtake your browser session including all authentication cookies or many other bad stuff.

If you are using puplic WiFi, I’d recommend a good on device firewall, or better just use a VPN to get an encrypted tunnel to your home (where you would need to open a port for that tho) and go into the internet from there.

#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.

Agreed. That's mostly BS from people who make commissions from some vendor.

#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?

A Firewall might be more advanced than just NAT/poking a hole, it may do intrusion detection (whatever that means) and DDoS protection

#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.

Maybe you've a bunch of IoT devices in your network that are sold by a Chinese company or any IoT device (lol) and you don't want them to be able to access the internet because they'll establish connections to shady places and might be used to access your network and other devices inside it.

#5 is the only one that makes some sense;

Essentially the same answer and in #3

If we're talking about your home setup and/or homelab just don't get a hardware firewall, those are overpriced and won't add much value. You're better off by buying an OpenWRT compatible router and ditching your ISP router. OpenWRT does NAT and has a firewall that is easy to manage and setup whatever policies you might need to restrict specific devices. You'll also be able to setup things such as DoH / DoT for your entire network, setup a quick Wireguard VPN to access your local services from the outside in a safe way and maybe use it to setup a couple of network shares. Much more value for most people, way cheaper.

It's to stop Genghis Khan from invading your computer.