In my experience, I only worry about PPMs when I’m growing in a straight Coco grow. The reason is Coco is neutral, unlike soil which has amendments in it to feed the plant.
With Coco you HAVE to add the nutrients or the plants will starve, which is why Parts Per Million (PPM) is measured in this case. It measures input of nutrients (food) and output (excretion of food). It’s how I determine if my plants are eating too much, too little, or if it’s just right. Like the three bears, ha!
I don’t do this with soil because, I shouldn’t have to add soil contains nutrients already and the numbers would be making me stressed because the numbers would be all over the place BECAUSE it’s soil full of food that I might be tempted to wash out.
With Coco, I can always correct if I feed too many nutrients by flushing them out and letting the plants recover, and starting over. With soil, this flushing business would be detrimental.
Long story short, one of the reasons is PPMs are measured to understand nutrient digestion or lack thereof of plants.
I hope that makes sense.