There are lots of times when you don’t want the lights on at full brightness. An example is a long length of red ropelight you might use to show a “do not cross” boundary around the perimeter of the light display. If you dim this ropelight you’ll save about 50% in energy.
There are several dimming techniques. You can always go to the home improvement store and ask for a lamp dimmer. Be careful. Most of these will only handle 300-600 watts. If your dimming load is higher you’ll cook the dimmer.
You can use the professional stuff called DMX and control the dimming by a computer. This is expensive hardware and can typically handle high current demands. Pro Sound and Stage Lighting is a good place to start. It helps if you already have a theatrical background because DMX can get pretty technical in a hurry.
You might also consider components from Light-O-Rama since they are geared more towards the Christmas light enthusiasts of the world.