Dear Student
Standard units are used so that there can be uniformity throughout the world. Other units such as a hand span, etc are cause confusion because they vary from person to person.
The internationally accepted standard units are the SI units. The International System of Units, universally known as the SI (from the French Systeme International d’Unites) is necessary because lack of standard unit of measurement would cause confusion and a waste of time in converting from one unit to another all the time. So, the SI system is adopted all over the world as the standard measurement unit system.
Regards