The USA is the only major country that has retained a non-metric system. The metric system was developed based on scientific principles to meet the requirements of science and trade.
Advantages of the Metric System
Simplicity.
The Metric system has only 7 basic measures. The imperial system and the USA system have over 300 different measures (see http://lamar.colostate.edu/~hillger/traditional.htm), of which many are ambiguous.
Ease of calculation.
All the units in the metric system are multiplied by 10 (to make larger units) or divided by 10 (to make smaller units).
International Standard.
With the exception of the USA, all major countries have converted to the metric system.