In certain locations, underwater lights are carried on all dives and, in other locations, only for night dives. These lights range in price and style, and you need to decide if your type of diving warrants purchasing an underwater light. Lights are useful even in clean water because they intensify colors when shining on the reef, in shallow depths, or in underwater overhangs.
There is no dive-related technology (other than personal dive computers) that has undergone remarkable changes like underwater lighting has enjoyed in the past 10 years or so. Dive lights are more reliable, smaller, more powerful, and cheaper now than ever. A good dive light will bring you lasting benefits and is a useful tool when diving in a remarkable range of conditions.