They can be a useful training tool for dryfire practice. Being able to keep that dot perfectly still (on the wall) as you pull through the trigger is like a high-tech version of the "dime" drill.
They can be useful for officers using shields where they cannot get the gun up into their line of sight, or even properly shoot from retention.
Otherwise, they are a whole lot less useful than the advertising makes them out to be.
I have a friend who did a very long-term, very scientific test of shooters of various skill levels as they learned and trained and used their handguns, both with traditional sights and with lasers. The laser-equipped were never able to get even somewhat close to either the speed or rapidity of accurate aimed fire as they could with iron-sighted weapons.
It has to do with the process by which we learn to see the front sight imposed on the target, and are able to track it as it returns to the target. The "bouncing ball" of the laser sight goes way off track during recoil (because it is at the end of an infinite line the angle of which is sweeping through many degrees of arc rather than a fixed object on the end of the slide, no farther away than the end of your reach). That has shooters NOT seeing "
front sight, press, front sight, press" but peering over and around their sights trying to locate that little red dot on Mr. Bad Guy (or the wall behind him...
).
Some shooters feel that the laser helps them aim "difficult" guns better, or shoot under conditions they otherwise could not. But these are, generally speaking, training issues that the laser cannot truly solve.