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There are many lighting options available for lighting your tanks. If you just have fish, and no corals or anemones, you don't need high intense lighting, but if you are going to keep corals, or anemones, then you need to have *good* lighting.
Time and time again, stores have sold customers lighting that was less than adequate. Why? Because they want to make a sale. If they can offer the customer a cheaper light, even though it is not good enough, but make a sale then they often will. The common phrase is "This light will be plenty of light for a reef tank". Then you get your tank, you think that the store down the road that quoted you a more expensive light was trying to rip you off, and you start stocking your tank with corals, but after a while you start having trouble keeping some of your corals, but can't understand why. Eventually you learn the hard way, that your light was not the intensity that you should have gotten.
So how much light should you have? You should have 3 to 5 watts of light per gallon, but 3 is quite low. Often times stores will sell lights that give you around 2.5 watts per gallon. 5 is more where you want to be, and even more is better. I like around 6.6 watts per gallon.
Higher light means healthyer corals. Even if a coral is slowly growing under low light, that does not mean it's fine. Under low lighting a coral is has less vigor, and is more at risk of getting an infection and dying. A coral growing under high intensity lighting will have more vigor, will grow faster, and if damaged will heal up much better. Lighting is everything in a reef tank. In fact with my own personal tanks, if I'm setting up a reef tank, and it won't fit 400w metal halide lighting, then it's the wrong size tank for me. Metal halide lights rule. There are lower wat metal halide lights available as well.