Can someone please ELI5?
Fast forward a couple of decades and everyone needs 10 IPs each. You have your phone, your laptop, your work computer, your TV, your door lock, your door bell camera, your thermostat, etc. And every web site in the world traditionally needed its own IP. And so the world pretty much ran out of those 4Bn addresses.
The "Powers That Be" developed IPv6 which solves this, but not everything works properly yet when connecting with IPv6, so if you want to make sure your software/hardware is guaranteed to connect to everything then you need one of those precious IPv4 addresses.
Now, in the early days of the Internet there were so many addresses that they were handed out like Halloween candy. And many people had so many they didn't even use a fraction of them. So now you can make good money selling your old addresses as they are prime real estate.
Your phone perhaps, but the rest of these devices never need a public IP address.
Pretty much every cell network gives the phone an IP on the subnet, and then uses NAT, or CG-NAT[1] to share the same public IPs for multiple mobile devices.
We can debate whether that's a good thing or a bad thing, but that is the way IPv6 is supposed to work.
https://en.wikipedia.org/wiki/Thread_(network_protocol)
Some IoT devices are now IPv6-required.