[mikeash]

Both increasing and decreasing capacity is expensive. If people use more power, you need to build more power plants, and that costs a lot of money and is politically difficult. If people use less power, you now have excess capital investments in the form of power plants you built in the past.

Their ideal is for power usage to remain exactly constant, or at least grow at a constant pace that matches local realities. The population and their power use wants to grow faster than that. Efficiency helps to compensate for that.

You'll also note that a lot of the efficiency tips are aimed at peak consumption. That's not the case for light bulbs, but there's a lot of stuff around more efficient air conditioning. Peak power is much more expensive than average, and in many places the power company can't charge residential users accordingly. Air conditioners tend to be used at peak usage times, so decreasing that peak usage can save them a lot of money.