Prior to the Revolution, in France alone there were allegedly 250 thousand various units in use! (All sorts of units, not just length.)

It didn't help that pre-Revolutionary France was a political Frankenstein stitched together from dozens of regions with completely different history (Celtic Brittany, Flemish Dunkirk, Germanic Alsace, Provencal South, Catalan Roussillon, Italian Nice) and thus very different local standards of everything, including measurements and law.

Unification of units removed a massive constraint on international trade and engineering. Except the US and Myanmar, of course... it is so frustrating to order anything from Myanmar e-shops, I must say. But Myanmar is at least promising to move on.