Rail-mounted laser units for pistols are by now fairly commonplace, and with good reason. They allow for very accurate shooting under low-light conditions where your front sight may not be clearly visible. They also feed into the reality that under stress many shooters will take a target-focused approach to self-defense and instinctively point shoot. With a laser attached to your gun, point shooting makes a lot more sense—not to mention the effect a bright laser dot has on would-be assailants.
There are many anecdotal examples of ne’er-do-wells losing heart and giving up the fight very quickly when they see that distinctive bright dot on their person. There are few visual cues more effective than that at conveying certain and imminent demise.
Tactical rail-mounted high-output lights serve a similar function but perhaps without the pinpoint accuracy of a laser. Tactical lights are excellent for low-light conditions, help properly identify your target and can have a blinding effect on assailants, as well as disorient them when equipped with a strobe function. Even better, many manufacturers offer both a tactical light and laser in one unit.