One reason why I’m a great fan of the metric system is that I know a tiny bit about the complete mess that preceded it.
These days people tend to think the choice is between metric and imperial, but before the French Revolution each country had its own system of measurements.
If this situation had been continued till the present day, just imagine how the signs indicating a speed limit of 80 km/h would have looked in different countries (see the illustration on the left). Every time you crossed a border, you’d have to learn new measurements.
It’s so much saner to use the same system everywhere, and I feel the UK and especially USA have broken the unwritten agreement by sticking to the old system while benefiting from other countries’ sanity.