If the system can do a job with smaller power by utilizing precision targeting then you can get away with a system that does not use too much power and much supporting systems for the high power.
Because the less power you have, the less range you have which gives you less time to down your target, as the laser has to stay on target to set it on fire in the first place. If you can engage targets from 5km, you will have a better chance of destroying it before it destroys something important. And, range also improves your chances of dealing with one target and switching to another, which matters a lot, just look at Russia launching dozens of cheap drones with each of their missiles now.
And on top of that, less power you have, the more time you need to destroy your target, because again, laser has to stay on target until it gets damaged by the heat it creates; which becomes a problem when you already have shortened time window if you have a less powerful laser. Which circles back to chances of destroying or being destroyed and ability to switch to next target as fast as possible.
And, with a less powerful laser, the temperature you can reach also drops, which means effectiveness of the laser against targets made from different material might be lower. So, you might (again) need more time, or might not even be able to damage it in time. Hence, why having a more powerful laser, if you can indeed have it, is much better than just sitting on your arse with a less powerful one.
Not to mention, all laser systems has to be incredibly precise, they have to be able to stay on targets that are traveling at hundreds of kms, how much more precise do you think they can get?