Its just a matter of efficieny and using your brain, especially if you have aoe heavy dos in group and one that is not aoe heavy, but single target heavy and due to that alot more efficient against that to bring down targets with high health quick down to low health so that everything with low health can then be cleaved to death by aoes


Its just the smarter way of playing, unless the low health target is top priority to kill asap
Smart players do always use their most potent attacks against targets that have high health, because using them against targets eith like 5-10% left would be wasted potency

As a machinist you will quickly learn to constantly switch your targets and between priorizi g single target rotation over using aoes, because you are more effective with single target skills, if there are less than 4 targets sniping one down after another, than to use aoes that are weaker and when you know, the aoes of say for example the dancer in the group will kill anyways near desd targets, so its better for you to focus then on high health targets