Why is it useful / necessary? To make the lamp burn softer, if a bright lamp needs to be able to read, but you still want to have a cozy atmosphere for an evening to celebrate a birthday and the like. Isn't it nice to have a bright lamp on? You can therefore turn the lamp down by turning the knob. You can also turn your lamp brighter again. This lowers and / or increases the voltage of the current. You also save energy by letting the lamp burn softer. So that it doesn't burn all day long. Which lamps are dimmable? The LED lamps are almost always dimmable, incandescent and halogen lamps are only dimmable if the dimming symbol is on the lamp or packaging. It is also useful to check that with the LED lamps. Lower limit of dimmer It is important to see how many watts the dimmer can dimmer, for example if you have a dimmer, whose lower limit is 20 watts, and you want to go 4 lights of 3 watts dimmer, you end up with 12 watts so that is not enough then the dimmer is not suitable for that.