OEMs build devices for specific carriers, countries, and regions. Not all devices of the same make and model are built with the same modems and other internal hardware. The result is that a small nuance in hardware can have ramifications on either the device or network, ranging from a poor user experience to a device that simply will not activate on a given carrier.
This issue is markedly more of an issue with Android devices but does impact Apple devices as well. When buying and selling devices all over the world, it is important to know if certain devices will work on specific networks.