Think of electricity as a handy on-demand way of moving work done in the generating plant into your home's lights and appliances.
If you turn off the switch, you break the circuit, and your lights no longer request this work of the generators. That means they consume a little less fuel, and in the aggregate if a lot of other people go to bed too, some of the generators could probably be shut down for the night.
Of course reacting to demand changes is itself a challenge. Some types of generators can be started or stopped more readily than others, so they make the adjustments with those first. There are also some ways to store excess generating capacity at one hour for use at another - pumping water up hill (that can later generate power running back down), charging huge battery banks, spinning up flywheels - all expensive and having their inefficiencies, but in use or contemplated to some extent. The other approach is to try to activate and de-activate various industrial power uses that don't have to be done at a particular time, to use the capacity of generators when they are under-loaded, and not use it at the peak of a summer day when all those air conditioners threaten to bring the grid down.
As for the power dissipated in the wires, at least for well-behaved loads that basically scales with the amount of work they are moving from one place to another, so when you turn off the light, most of the power lost along the way in moving that work to you stops being lost as well. The only degree to which turning off the light is not effective comes from the fact that a practical generator's fuel consumption is only loosely correlated to its electrical load, and the overall operating expense comes not only from usage, but also from having it available to use. And secondarily, to a very tiny extent, the miniscule power lost to imperfect insulators, induction, etc by having a grid of power lines, transformers, and compensating capacitors cycling at 50 or 60 Hz regardless if they are loaded or not.