SIM Authorization?

Jun 30, 2013
Can someone explain how this works?

I was using my phone in the US fine with H2O Wireless and am now visiting the UK for a week. I bought a local SIM card, but when I inserted it, got a message that the phone is SIM card locked. Took a while to figure it out, but I had to get on to WiFi, login to my Xiaomi account, then I saw a message that it would re-authorize my phone for the new SIM card. Now it works!

What the heck just happened?!? I figure this is some kind of security but I can't understand the benefit of it.