LED = light emitting diode, meaning that they are designed so that electricity will flow through only in one direction. If polarity is backwards, electricity is "trying" to flow the wrong way causing them to not light up. Although not really a necessity to having the system up and running it is nice to know that they are working when you have some diagnosing to do. My BIOS once showed my old HDD as my master, even though I had put it back in the old system. It was extremely helpful to see at POST that the hard drive light wasn't flashing so that I knew that it wasn't being accessed.