why were si units developed
Answers
As the name indicated "International" - before the development of SI units, nearly every country would have developed their own "National" units of measurements. Whilst that would be fine when used within their own country. But International Trade happened.
With people trading internationally, the need for an agreed set of measurements of units to be referred to become essential. Otherwise country A may have provided their "tonne" of bricks (according to measurements in Country A) to Country B which only perceive it to be not quite their "ton" (according to measurements in Country B).
Having different measurement standards throughout different country makes it too complicated to trade.
Thus, it makes sense to have 1 standard for every country to refer to.
The International System of Units, universally abbreviated SI (from the French Le Système International d'Unités), is the modern metric system of measurement. The SI was established in 1960 by the 11th General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures).