They develop a laser to do job that relatively small caliber autocannon can do.
Benefit of this, to a solider, is there is no difference in elevation between engaging a target at 20 meters out to say 700 meters. This will enable them to be far more effective in urban battle fields as they can engage targets far more quickly and effectively.
"HELIOS...is designed to track and destroy small, unmanned aerial vehicles or boats that approach a ship."
Though these lasers are capable of downing small UAVs, they're really intended to counter small boat swarms (Boghammer type vessels) favored by Iran.
The HEL demonstration with 15 kilowatts laser took forever to ignite small fire in a target boat (directed at a single point in the outboard motor). 150-kilowatt laser is ten times faster but simple water pump in the front of approaching boat is effective countermeasure.
DARPA has had low power laser demonstrators for 20 years. They are cheaper now but developing yet another is not going to produce more improvement in the power scaling of lasers. High power lasers are hard for optics and all other parts.
They can also have applications in high-altitudes and in space. Maybe to take down MRVs? That makes a lot more sense actually.
You need very high power lasers for that. These are the hopes DARPA has for lasers but they don't seem to materialize.
All of these make lasers very compelling for defensive CIWS.
The fundamental problem the Navy is terrified about is huge barrages of high speed, high maneuverability, anti-ship missiles. Existing systems are very limited in stopping such an attack if large enough.