> Why? Why not connect [the Raspberry Pi] to the network you want so that it just connects to that going forward?

I'm not the guy who wrote that, but I had the same use-case myself. (Except that I happened to choose the correct networking stack so I didn't have a problem). I wanted to set up a Raspberry Pi in my parents' house that would run Tailscale so I could use it as an exit node. (With my parents' full knowledge and permission). I wanted to pre-configure it with their WiFi password so that when I showed up for Christmas, I didn't have to spend any time configuring the device, just plug it in and go have dinner. (Then they changed ISPs, got a new router with a new WiFi password, and I had to ask them to plug it into the wired network so I could connect to it remotely and change the WiFi password again, so I had to do that work twice. But thankfully, I didn't have to walk them through the steps, just say "Hey, please plug it into the router with an Ethernet cable until you get an email from me telling you I've reconfigured the WiFi".)