What do lan_dev and wan_dev statements do?

Installing and Using OpenWrt Network and Wireless Configuration
1

In the starting configuration of LEDE (at least on erx), in /etc/config/network I see two statements:

config device 'wan_dev'
option name 'eth0.x'
option macaddr 'x'

config device 'lan_dev'
option name 'eth0.x'
option macaddr 'x'

They set MACs for the interfaces, I've noticed that setting wan_dev changes the interfaced showed in LuCI (not really, I haven't noticed I've named new interface wan... ) but what more do they do, especially lan_dev (and how to use it especially in case where I have more then one lan/vlan)?
I can't find no documentation for those options/devices.

2

These are default mac addresses extracted from eeprom set to interfaces. On ath79 platform the not always can be set in kernel. So it is done in userspace.

You can leave them, or set mac addresses for your real interfaces.