Related query:
Is there any reason why a router needs multiple hardware interfaces?
I don't see any reason why I can't have multiple ADSL routers all with static IP addresses on (say) 192.168.1.x addresses and the rest of the network on 192.168.2.x addresses, all on the same ethernet network, but that doesn't seem to be how things are expected to be set up and I can't get my head around why it matters.