Why is PPM a useful measurement?
It's often used to measure contaminants, but there's other applications.
When you read about an article about air pollution or water pollution, you'll often see it refer to the concentration of contamination in ppm.
here's a NASA article talking about the concentration of CO2 in the atmosphere reaching 400ppm.
You can also get a water quality tester to see the concentration of foreign particles in the water.