Some plants only thrive and to their thing in really high light levels. However, it can be tricky to provide all the light required without introducing excess heat problems as a side-effect. Sure, the closer you position your grow lights to your plants, the more intense the light—but along with this increased intensity comes increased heat from the lamp!
So what's the answer? In three words: Air-cooled reflectors! Using an inline fan and ducting, growers air-cool their lamps and afford themselves a crucial extra few inches when it comes to positioning their grow lights to achieve a truly intense level of incident energy hitting their plants' leaves. Meanwhile, up to 50% of the heat is vented away thanks to the air-cooled reflector.
Air-cooling doesn't remove all the heat, and there's still the infra-red heat generated from the lamp to consider - but if you're growing plants native to higher altitudes (where they receive plenty of sunshine but at relatively cool temperatures) air-cooled reflectors are definitely the way to go!