A valuable tool which provides unique capabilities. A laser sight is a device which produces a very narrow beam of light which can be "sighted in" to a gun, so that the beam hits where the round hits.
Its purpose is to show the shooter where his gun is pointed, as a means to aim his gun. It can also show others where the gun is pointed, which can be useful for intimidation, communication.

A gun equipped with a lasersight you can simply move the red laser dot onto the target, then fire. It is no longer necessary to close one eye and use the conventional sights. The choice of usinng lasersight rather then the use of conventional sights greatly increases shooting speed and accuracy. The lasersight only gives certain advantages:

Advantages

You can shoot from the hip with the same accuracy as from the shoulder, but much faster. In any situation when you are on the move, rather than behind cover, first round accuracy while firing quickly from the hip may spell the difference between a hit or miss.

With the gun at the hip or underarm position, you can track a moving target or shoot while on the move more effectively than when sight a target through conventional sights.

You can aim accurately in dim light when it is difficult or impossible to see your conventional sights. The lasersight performs beautifully in conjunction with Night Vision.

You gain a psychological advantage over your opponent by putting the laser spot on the target and making certain that the target is aware of it. Seeing the dot, the target knows exactly where he will be hit.

Since there is no need to raise the weapon to eye level, you retain unobstructed peripheral vision while you fire. This reduces the risk of hitting your teammate who crosses the line of fire.

Provides an independent check on where the weapon is aimed at any given time.

You can use the lasersight for many purposes other than aiming a gun. As a pointer, it is an extension of the index finger to designate a target or an area which one should take cover. The lasersight can even be used for secure signaling between you and your teammates

Disadvantages

The use of a laser sight to aim your gun(s) certainly appears appealing and a lot easier than developing the skills necessary to use your gun's conventional sights effectively.
A laser sight should be used as a tool, in certain situations such as described above, and not as a replacement for your gun's conventional sights.
Over fifteen years of laser sight use has shown that they are "slower" and less "reliable" than conventional sights for skilled shooters.
The reason is simple. It takes too long to "find" the laser dot on your target in all but the most ideal ambient lighting conditions.
In contrast, you know where your sights are on your weapon, and you can develop the skill to bring them to bear on target much quicker than you can find a laser spot on a target.