As @XRaySpeX says its for the optical line termination unit (ONT), which the router will connect into using a traditional network cable. The ONT needs power to convert the optical into electrical signals and vice versa.
Because this is a new line, the engineer can route the fibre cable into your home at a different location to the existing BT socket. Although it is helpful if the new location is easily reachable from where the existing BT duct surfaces outside your house, as they will (generally) externally route the fibre to the new entry point.
I would take advantage of the opportunity to have the optical fibre routed to a location which is more convenient (ie. adjacent to a pair of electrical sockets) and positions your router more central to the house and so improve WiFi coverage.
