Unlike some non-metric measurements, which can vary even between countries, metric measurements are set by international agreement and therefore are the same wherever you happen to be. You can be confident that a metre, kilogram or litre will be the same whether you are in the UK, Canada, France, Germany, Australia, Russia, Israel, Egypt, Jordan, South Africa, or USA, etc. This has commercial and trade advantages for all businesses who trade internationally, as only one set of measurements is needed.
The metric system is used by 96% of the world's population and by all countries for scientific use.
Because the metric system is a decimal system of weights and measures it is easy to convert between units (e.g. from millimetres to metres, or grams to kilograms) simply by multiplying or dividing by 10, 100, 1000, etc. Often this is just a case of moving the decimal point to the right or left. Fractional notation is not used, thus making any mental arithmetic very easy.
Calculations of metric measurements are easy to perform and price/weight comparisons easier to understand.
As multiples of units have a standard prefix, it is easy to see how much a unit is of another unit, such as when you see the kilo- prefix, it means one thousand of something, e.g. 1 kilogram is 1000 grams; see section on prefixes for more info.
When working with just one measurement system, it is much less likely that errors will occur, such as with machinery, manufacturing, NASA space probes, etc.
In the future, the metric system is likely to prevail as the one and only measurement system used throughout the world, and so those who do not use it or continue to use non-metric units will be left behind the global economy.