The fact that converting measurements is trivial in theory doesn't mean that you don't have those measurements embedded in physical artifacts in non-trivial ways.
Any sort of machinery that deals with measurements, has some sort of "standard" embedded in it.
I'm a woodworker, so I'm going to pick on the world of woodworking.
A thickness planer (thicknesser if you're from the UK) dimensions wood to a consistent thickness. The adjustment is typically done by raising or lowering the bed relative to the cutterhead. There is generally a crank or handwheel that turns some kind of gear or screw mechanism that in turn moves the bed.
In the US, woodworkers generally use inches, and work in thicknesses that are some even number of 16ths of an inch.
It would be possible, in theory, to set any planer for any dimension in the continuous range of dimensions it supports. In practice, the adjustment mechanism is set so that a whole turn of the crank corresponds to some whole number of 16ths. Moreover, this is usually set up so that a whole number of 16ths lands at an easily repeated position: crank handle at 12 o'clock or 6 o'clock. This makes it blindingly easy to repeatably hit the same mark (within woodworking tolerances) every time you use the machine.
So far, so good.
The shop I belong to where I have access to a planer has a modern Powermatic planer. Powermatic no longer manufactures in the US, and the planer was a bit of a source of mystery to me until I dug up the docs and read them. One full turn of the crank is 1.5mm. I'll spare you converting: That's 1/16th of an inch, less 3.5 thousandths of an inch (thou, rhymes with cow).
That doesn't sound like much, but it adds up. Over 16 turns of the crank, you're now 56 thou off where you expected to be, or, almost exactly 1/17th of an inch[0]. Moreover, hitting 3/4" exactly (within woodworking tolerances) requires a bit of guesstimating about how much extra you need to turn the wheel past 12 o'clock or 6 o'clock.
Non-solutions to this problem:
1. Put a measuring device on the machine. They either aren't accurate enough, don't stay accurate, or cost a fortune (i.e. anything digital that is both accurate and stays accurate)
2. Work in metric. I would, happily, except for the fact that literally everything else in the shop is inches. Chisels in the US are an even number of 16ths of an inch wide. A dado stack for a table saw cuts dadoes an even number of 8ths, 16ths, or 32nds of an inch wide. The simple act of finding a measuring tape that's metric is a pain in the ass. I can go to Home Depot, Lowes, or any hardware store and find a dozen options in inches. If I'm lucky, there's ONE with metric, and it has inches on one side of the tape.
But wait, it gets worse if you're a metalworker. Metal lathes have a leadscrew that rotates a fixed number of rotations per rotation of the workpiece (non-continuously variable by gearbox). Those are threaded either in either metric or inches. To do metric work on a lathe that's natively inches (or vice versa), you need gears in the ratio of 254:100, reduced to 127:50. If that sounds like a lot of teeth on one gear, that's because it is; making that set of gears requires either an impractically large gear (won't fit on the lathe), or impractically small teeth (incapable of transmitting the required torque)[1].
Once you start making actual physical things, you find that your system of measurement is embedded in nearly everything around you in ways that are difficult to work around while maintaining sufficient accuracy and precision.
[0] Yes, this means that 1.5mm is almost, but not quite, exactly 1/17th of an inch.
[1] There exists a very close approximate solution that can be used subject to limitations you'd need to find a machinist to expand on.
When I did a little home improvement project I always thought what a PITA it is to multiply 5 and 3/8 inches seven times. Let alone divide that number by 3. Metric is so much easier.
Any sort of machinery that deals with measurements, has some sort of "standard" embedded in it.
I'm a woodworker, so I'm going to pick on the world of woodworking.
A thickness planer (thicknesser if you're from the UK) dimensions wood to a consistent thickness. The adjustment is typically done by raising or lowering the bed relative to the cutterhead. There is generally a crank or handwheel that turns some kind of gear or screw mechanism that in turn moves the bed.
In the US, woodworkers generally use inches, and work in thicknesses that are some even number of 16ths of an inch.
It would be possible, in theory, to set any planer for any dimension in the continuous range of dimensions it supports. In practice, the adjustment mechanism is set so that a whole turn of the crank corresponds to some whole number of 16ths. Moreover, this is usually set up so that a whole number of 16ths lands at an easily repeated position: crank handle at 12 o'clock or 6 o'clock. This makes it blindingly easy to repeatably hit the same mark (within woodworking tolerances) every time you use the machine.
So far, so good.
The shop I belong to where I have access to a planer has a modern Powermatic planer. Powermatic no longer manufactures in the US, and the planer was a bit of a source of mystery to me until I dug up the docs and read them. One full turn of the crank is 1.5mm. I'll spare you converting: That's 1/16th of an inch, less 3.5 thousandths of an inch (thou, rhymes with cow).
That doesn't sound like much, but it adds up. Over 16 turns of the crank, you're now 56 thou off where you expected to be, or, almost exactly 1/17th of an inch[0]. Moreover, hitting 3/4" exactly (within woodworking tolerances) requires a bit of guesstimating about how much extra you need to turn the wheel past 12 o'clock or 6 o'clock.
Non-solutions to this problem:
1. Put a measuring device on the machine. They either aren't accurate enough, don't stay accurate, or cost a fortune (i.e. anything digital that is both accurate and stays accurate)
2. Work in metric. I would, happily, except for the fact that literally everything else in the shop is inches. Chisels in the US are an even number of 16ths of an inch wide. A dado stack for a table saw cuts dadoes an even number of 8ths, 16ths, or 32nds of an inch wide. The simple act of finding a measuring tape that's metric is a pain in the ass. I can go to Home Depot, Lowes, or any hardware store and find a dozen options in inches. If I'm lucky, there's ONE with metric, and it has inches on one side of the tape.
But wait, it gets worse if you're a metalworker. Metal lathes have a leadscrew that rotates a fixed number of rotations per rotation of the workpiece (non-continuously variable by gearbox). Those are threaded either in either metric or inches. To do metric work on a lathe that's natively inches (or vice versa), you need gears in the ratio of 254:100, reduced to 127:50. If that sounds like a lot of teeth on one gear, that's because it is; making that set of gears requires either an impractically large gear (won't fit on the lathe), or impractically small teeth (incapable of transmitting the required torque)[1].
Once you start making actual physical things, you find that your system of measurement is embedded in nearly everything around you in ways that are difficult to work around while maintaining sufficient accuracy and precision.
[0] Yes, this means that 1.5mm is almost, but not quite, exactly 1/17th of an inch.
[1] There exists a very close approximate solution that can be used subject to limitations you'd need to find a machinist to expand on.