Question

why do we need S.I units for measuring physical quantities like length weight etc

Answer 1

when SI unit are not happen then the earlier people used CGS OR MKS system or hadspan system which does not give appropriate answer or unit so after a period SI unit will be devloved to find the unit of physical quantity i.e., FORCE=Newton,length:metre and weight: kilogram or kg in before the unit or the CGS unit of force is Dyne or CGS unit of length is centimetre which does not get clearly correct answer...