Ability to make changes to one segment of the network without affecting the other. Upgrade your gateway device/router separately from wifi. Put multiple APs all controlled by a single unifi controller around your house, rather than mess with half duplex csma hell wifi extender devices.
Have a router supporting weird configurations like CenturyLink gigabit gpon service, which if you don't want to use their terrible gateway device, requires a unique vlan tag on the wan interface.
Ability to have a router that is Debian under the hood.
Just curious, why? (You just mentioned "shoulds" and "dos", not "whys")