In short, you always take a unit so that your values are around 1, because they are the number that make sense to us and are easier to compare. In subatomic, you even transform everything so your references are just 1. Weird at first, but powerfull to do calculation easily.

That's why physicist always come up with new units, even though they are just derivative of common units (planck length, time, angstrom, nm, micrometer), To each unit it's reference:

• angstrom: atom

• meter: human

• kilometer: geographical distances (you don't say that Paris is 800 000 meters from Bordeaux, right?)

• AU: solar systems

• parsec: galaxies and beyond (with kpc and Mpc)