If a kilo, for example, was made of 12 x 12 x 12 (1728) grams instead of 1000 grams, then you could sell stuff by the half-kilo, third-kilo, and 1/12 kilo without going into decimals. The number 1728 would still be written as 1000 in base 12, but 1/3 kilo would be 400 grams instead of 333.3333... as it is now. Useful if you're a grocer.

Other applications, like documenting how much oil to put in a car or whatever, there would be more options for picking memorable numbers. Like, my motorcycle takes 1.6L of oil. It could be 1 represented as 1 2/3 L, which is more visually intuitive. Not sure if I explained that well enough, maybe someone else has got a better metaphor handy...

Is it some American thing that measuring things as fractions is easier than decimals? To me being used with metric system 1.6L is equally intuitive as 1 2/3, maybe even more intuitive.

It's just hardwired in the brain how much 0.6, or 60%, represents, I think internally my brain finds the closest easy references like 10%, 25%, 33%, 50% or 75% and then interpolates or adds or removes chunks of 10%, the rest is rounding errors. I mean, i dont have to think consciously if 0.69 is more or less than 75% and i know exactly where both 66.66% and 75% is and can place it somewhere in between.

I agree that fractions are a very American thing. A lot of stuff is expressed as fractions here. But I bet a lot of people couldn't tell you whether 5/16 is more then 1/3 or not.

Another thing is that multiplying and dividing by 2 and 5 is easier in base-10. For example, to divide by 5 you multiply the number by 2 (just add it to itself) and then divide by 10 (a right shift).

On the other hand, to divide by 3 you basically need to do long division.