history
As science developed and measurements and manufacturing became more precise, the characterization of error less than 1% when expressed in % had a lot of leading zeros. In order to make the numbers more manageable, parts per million were introduced. When things became more precise parts in 10^n type formats were introduced.
This measurement can also be used to show the ratio of a small quantity mixed in a larger quantity, either by number of molecules or by weight. Gas detectors are rated in the ppm concentrations they can detect.