But it doesn't use base 12. Take distance. Values smaller than 1/64" are measured using "thou", "tenths", and "millionths", which are decimal multiples of 1/1000', 1/10000", and 1/1000000" respectively.
Values between 1/64" and 1" are measured using dyadic rationals, i.e. base-2 fractions.
It makes it easier to divvy up into groups: halves, thirds, quarters, sixths (vs. just halves and fifths). Makes doing math easier in the head (fewer digits, checking for divisibility are the big ones).
Also, on the practical level, I have a specific mark to go to for 1/4". 1/4cm involves me guestimating the middle between two mm marks and just deciding that that's middle enough. Small errors like this can actually add up really fast in something like woodworking
And why are there 16 cups in a gallon, 15-and-some tablespoons in a cup and 3 teaspoons in a tablespoon?
Better make it 12 tablespoons in a cup and 12 cups in a gallon, then!
And why are there 14 pounds in a stone and 16 ounce in a pound?
The imperial system does not use dozenal.
It uses a clusterfuck of bases because it's actually a clusterfuck of measuring systems in a really big trenchcoat