If you travel internationally, it’s nice to be able to put in a local-country SIM card for Internet and calls. This is far cheaper than International Roaming.
AT&T won’t do it
I just confirmed with AT&T that they changed their policy. They no longer unlock the SIM card on iPhones, even for customers in good standing for over a year. Compare this to Verizon that will unlock a phone after 3 months.
So if you are buying an iPhone and travel internationally, don’t chose AT&T as your carrier.