One of the advantages would be the speed. An email can be delivered to the recipient a lot faster than a letter could be. It also doesn't cost (apart from your monthly Internet bill) :)
Because the units are multiples of ten, they can easily be calculated using our base-ten number system, and expressed in interchangable terms by simply moving the decimal point (no calculation is...
Well... without number systems math would be impossible... and the peoples without this will not really cooperate with each other in every moment and without this the world is useless.