Luca gave me the OK to chip in here so I'll throw in my two cents.
While yes, a particle beam, or plasma based medium would be the traditional way to add extra hurt to a weapon this technically doesn't need it.
Here is the deal: A laser weapon cuts out the middle man of a transmission medium (the particles, or plasma in a particle beam, or plasma weapon) instead it converts its entire energetic payload into photons (light) and sends them on their merry way to the target. What happens is when they reach a solid entity they dump all of their energy into what they hit. What this functionally means is that it converts whatever substance is at the point of impact into a cloud of plasma with a huge amount of energy resulting in an explosion more or less.
Now typically the issue with laser weapons is materials tend to reflect some of the photons depending on what it is and what wavelength, and at worst something with properties like zesuaium will just reject all of the transferred energy. In this case however in an atmosphere the energy gets dumped into the nearby air since it scatters off of the target which would cause a more mild, but still rather unpleasant blooming of plasma in the local air as the energy from the initially tightly packed photon beam distributes itself into the atmosphere. Of course in space this doesn't happen and you might as well be shining a flashlight at them.
So yes it has some downsides, but on most targets it'll still flash heat a section of whatever material it strikes and turn it into a nasty ball of plasma.
Edit: There are also the issues airborne particles like fog, dust, smoke, the ablation from the laser's own interaction with the target, that will diffuse a laser and rob it of its energy, but that is more circumstantial and not core to the operation of the weapon so I didn't include it.
More or less if you can clearly see your target it'll hit with its intended punch, however like all weapons its effectiveness is lowered by circumstances in the environment.