I looked at these gadgets some time back and I think it is a handy tool for when your gun accidentally goes out, for instance, when you drop or bump it while hunting.
As far as I can understand, they work as follows. You sight your gun in the normal way, like you always would have done, when you are satisfied you put the laser in and shine it on the target, it will never be on the place where your shots have been, but this then gives you a reference point, lets say your crosshairs are on the centre of the target, where you have just zeroed your gun, and the laser is off to the right and high. You then mark this reference point of the laser in relation to the crosshairs; you can form a mental picture, or make a little drawing in your hunting log. So when you are out hunting and you suddenly stat to miss everything and do not have ammo to waste, you can quickly do a check with the laser to see whether your crosshairs have moved. The laser will be at a different reference point in relation to the crosshairs for different loads in the same gun.
To check the quality of these lasers you put them in the barrel of the gun, or in a non-moveable stand, usually sold separately, shine it on the target, and then turn the laser, the spot on the target should be on the same place and should not move when you rotate the laser. The better the quality of the laser, the less the spot on the target will move.
I think it is a handy tool, but definitely a gismo and if you not into that type of thing it is probably not worth having.