Why do scientist use si system of measurements?

SI stands for the French système international. It is basically the international system

of units. The system was created to be a coherent consolidation of all the forms of

measurements. Because it is a universal system used by everyone internationally (except

Liberia, Myanmar, and the USA) it means conversions are usually not necessary.

Conversions between units also become easier because the system was designed

that way (notice how factors are nearly always powers of 10).