Why is it important to have the international system of units?

(a) It is an international standard - if all countries use the same units, it makes communication easier. (b) The metric prefixes make calculations easier. For instance, it is much easier to convert kilometers to meters (multiply by 1000, that is, move the decimal point 3 digits to the right), than to convert miles to feet (I can't even remember the conversion factor, but it is not a power of 10).