It starts out arbitrary but becomes useful once everyone agrees with it, like measuring distance in meters- which has gone through several definitions, none of

It starts out arbitrary but becomes useful once everyone agrees with it, like measuring distance in meters- which has gone through several definitions, none of which make much sense in terms of being based on a natural constant and using a nice round numbers as a coefficient. If four instruments are following the standard and one is a bit flat because "A-440 Hz is arbitrary" then the whole band sounds like a mess.