The claim is you need to be on the carrier’s mobile data network, the carrier gives you an IP address, then a website owner asks the carrier who is at that ip address and then the carrier gives the website owner the data that it has on you (your real name, the address where they send the bills, the phone number they assigned to you, etc)
For supporting technologies like wifi offload, VoLTE, etc the phone can be told to tunnel traffic back to the carrier network, even when using wifi. This is to support features like using wifi to complete voice call's, but could be used for IP mobility as well (keeping you're IP address as you switch access networks).
I'm a bit rusty as I've been out of the industry for a year now and didn't work on this directly, so I forget how the phone get's this configuration. I think it might be an APN setting to connect back to the ePDG when on other access networks, but I could easily be mistaken.
People would visit that URL using mobile internet...