Preheat
Some types of incandescent (filament) lamps should not be switched to full power from cold, and doing so can shorten their life dramatically owing to the large inrush current that occurs. To soften the blow to the lamps slightly, dimmers may have a preheat function. This sets a minimum level, usually between 5% and 10%, which appears turned-off to the audience, but stops the lamp from cooling down too much. This also speeds up the lamp's reaction to sudden bursts of power that operators of rock'n'roll-style shows appreciate. The opposite of this function is sometimes called top-set. This limits the maximum power supplied to a lamp, which can also extend its life.
In less advanced systems, this same effect is achieved by literally pre-heating (warming) the globes before an event or performance. This is usually achieved by slowly bringing the lights up to full (or usually 90-95%) power over a period of between 1/2 to 1 hour. This is as effective as a built in preheat function.
Read more about this topic: Dimmer