Q.Why do we need a standardised, internationally agreed system of units instead of letting every region use its own units of length, mass and time?

[!TLDR] Different regional units cannot be compared without error-prone conversion. [!ANSWER] A single internationally agreed system (SI) lets measurements made anywhere in the world be compared, reproduced and combined directly, without ambiguity or conversion error.

If every region used its own units — as genuinely happened historically, with the CGS, FPS and MKS systems all in simultaneous use — a measurement made in one system cannot be directly compared with a measurement made in another without first applying a conversion factor. Every conversion is an opportunity for a numerical error (a wrongly remembered or wrongly applied factor), and in high-stakes contexts this is not a hypothetical risk: the loss of NASA's Mars Climate Orbiter in 1999 is a documented case where a unit mismatch between metric and imperial units caused a navigation error.

A standardised, internationally agreed system such as SI removes this risk entirely: a length, mass or time measurement made in a laboratory in one country can be directly compared, combined in a calculation, or reproduced exactly by a laboratory anywhere else in the world, because both are using identically defined units. This is essential for international scientific collaboration, global trade, engineering standards, and safety-critical fields such as aviation and medicine.

[!ANSWER] A common, internationally standardised system of units (SI) is needed so that measurements from anywhere in the world can be compared, reproduced and combined without ambiguity or conversion error.

Argue from the practical consequence of NOT standardising — comparison requires conversion, and conversion introduces error/ambiguity — to the benefit a single global system provides.

• Giving a vague answer ('it's more convenient') without explaining the specific comparison/conversion problem that standardisation solves