Long Gun Lasers
There are a number of laser systems out there for the long gun. Generally they aren't necessary for most folks who have a red dot optic on their rifle or shotgun. There are a couple of exceptions worth noting, however….
1. Night Vision
if you are using a night vision device that is mounted on a helmet (generally the preferred method for using them) then you aren't going to be able to look through your red dot optic to aim. This is why IR laser systems exist, to allow for an accurate aiming reference without having to have a cheek weld on the weapon.
2. Gas mask/hazmat/chem-bio suits
The masks in these suits generally prevent a decent cheek weld, meaning you probably won't be able to use the optic/sights on your carbine. A laser system like the DBAL would be an asset here.
3. Sight-to-barrel relationship
If you have a weapon intended mainly for close quarters that has a big difference in sight to barrel relationship it may be to your advantage to mount a laser to help deal with close quarters work.
An example is a Colt 9mm SMG. I was using one in a shoothouse recently that had an Aimpoint mounted to the top of the carry handle. As good as the Colt is and as good as the Aimpoint is, the combination wasn't so hot.
The Aimpoint mounted on top of the carry handle left a great deal to be desired. It made for a really nasty sight offset that complicated close range work.
In that situation a Crimson Trace unit or a DBAL would have helped to deal with the sight offset issue and would have made A zone hits and headshots much easier at the close ranges found inside the shoothouse.
If you aren't in those categories you probably don't need the laser on your weapon. Generally the sorts of guys who are in those situations are paid to carry machineguns by some sort of government entity.