As for SI, I have never liked the 'non-thousands' submultiples e.g. centimeters. I think it's confusing. You have km, m, and mm. factors of 1,000. Seems reasonable. Just like we have F, mF, uF, nF, pF for capacitors.
I think the various units are there because people like 'comfortable' numbers; e.g. people seem to prefer 8.9 cm to 89 mm ('8' is a number between 1 and 9, the most preferred numbers -- because we can count them on our fingers?).
As for SI, I have never liked the 'non-thousands' submultiples e.g. centimeters. I think it's confusing. You have km, m, and mm. factors of 1,000. Seems reasonable. Just like we have F, mF, uF, nF, pF for capacitors.
I think the various units are there because people like 'comfortable' numbers; e.g. people seem to prefer 8.9 cm to 89 mm ('8' is a number between 1 and 9, the most preferred numbers -- because we can count them on our fingers?).