Machinists and engineers often prefer Imperial "mils" (thousandths of an inch), for instance. It's easy to convert from kilograms to pounds or kilometers to miles, but there's no convenient metric unit for expressing typical distances and tolerances used in mainstream machining. A millimeter is way too coarse, a micrometer is way too fine.
As a specific case, in electrical work, it's easy for me to specify "6 mil trace/space" attributes for a PC board design. Not so easy to say "0.1524 mm" or "152.4 microns." If I round my specification down to 0.1 mm, the resulting copper features will carry less current and cost more money. If I round it up to 0.2 mm, other physical and/or electrical requirements won't be met. So now I have to add at least one more sig fig, which is a pain in the neck for no obvious benefit.