It is sometimes useful to have a board-specific routine that can set the MAC address of a network interface. In our use-case it is for a board by Dresden Electronic that comes with a unique MAC for its 802.15.4 radio. But other use-cases (like using an EEPROM which you can buy with a unique mac address) are useful.

Link to our Dresden-Electronic module (which includes an ATMEGA RFR-2 with a 802.15.4 radio on chip):
https://www.dresden-elektronik.de/funktechnik/products/radio-modules/oem-derfmega/

When I mention an EEPROM with a pre-programmed MAC I mean something like this (available from different manufacturers, I've seen those used on cheap ARM boards (mostly running Linux) but they may end up in HW running RIOT, too:
https://www.microchip.com/design-centers/memory/serial-eeprom/mac-address-and-unique-id-eeproms

Of course other sources of MAC addresses are possible (configurable storage in an EEPROM, serial-number chip like one-wire, etc).