's don't work anything like light bulbs. There's no filiment to burn out. Switching them on/off will actually extend the life. But even running 24/7 they're good for up to 100/thousand hours or more.
On whether or not they are necessary. They aren't a requirement. They are relatively new to the reef environment and lots of tanks have been doing without them for years and years. But they are much less intrusive than regular lights, so things that only come out at night when you can't see them are now visible. And some folks say their corrals are growing faster, fish spawn more often, and shrimp molt more often with them on.
I've read that the blue LED
's match the natural wavelength of moonlight better than other colors. I don't know if thats true or not. But if you want other colors, they come in blue,green,red,yellow,orange,white, and spectrums not visible to the human eye. You can also get blinking and color change LED
's. There are high intensity and low intenesity, if you want more or less light. They all work the same electrically, so the listed instructions will work the same. I'm putting blue in mine, but that's my personal color preference. Although I would be interested to see what a dozen high intensity, blinking, color changing LED
's would do for fish : )