Most of us live in the real world, normally (ha ha!). That's the place where things can be measured objectively. With standard units of measurement, we can write specifications and have conversations that are shared, consistent, and repeatable. Standards of measurement are important. And this is good, because human perception is not so reliable. Depending on the context, an object 1 meter long may appear subjectively to be larger or smaller than its actual size until it's measured.
A meter in length is the same whenever and wherever it is used. A caveat is, we must refer to the same units of measurement when sharing numbers. In September 1999, NASA lost its Mars Climate Orbiter spacecraft because one team used English units ...
