The answer to your question is as simple as it is unsatisfying. Additional degrees of precision in construction are only as useful as your means to use them.
If you using your saw can only cut to an accuracy of 1/8 of an inch, than any precision beyond that point is lost as you are unable to actualize it
However if you are using a saw and you’re now at the point of your personal skill that you’re measuring to the inside or outside of the mark on a ruler, then it is likely time for you to graduate between more precise form of measurement.
https://youtu.be/qE7dYhpI_bI?si=HCtTbklCA18ZieCh
This video covers a lot of the interesting points around measurement and how we can never truly be perfectly accurate with any measurement of any non-discreet metric.
To give a real world example if you are off by a millimeter diameter when building a car engine cylinder it will likely fail.
In addition to what @Pons_Aelius replied, it is also used as a benchmark/flex for computers, as to who can build a beefy enough machine or good enough card to calculate more digits of pi.