The modem is part of the BT Openreach FTTC service, the router is suppled by Plusnet. In principal, the demarcation point of the FTTC service is the ethernet port on the BT modem. You can connect a single PC direct to the modem if you like but using a router is far more convenient.
Some users prefer to use their own combined modem/router (usually quite expensive devices for FTTC (VDSL2)) but this
may not be
officially within the terms of the BT Openreach FTTC regulations.
I use a Fritz!Box 7390 combined VDSL2 modem / router but keep the BTO modem available for re-installation just in case of any problem.
Eventually I'm sure there officially will be a lot more freedom on the choice of devices connected to FTTC.
A typical modem/router probably consumes more power than either the BTO modem or a standard router but is likely more efficient then separate devices (though the electricity cost is not really significant compared with the cost of the FTTC service).
Call me 'w23'
At any given moment in the universe many things happen. Coincidence is a matter of how close these events are in space, time and relationship.
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