Question

Why do scientists use the International System of units

Answer 1

so they can share a standard system of units.
Just look at the troubles we have in the US, with our feet, inches, chains, miles, acres, roods, tons, ounces, fluid ounces, gills, cups, pounds, slugs, hogsheads, barrels, pecks, bushels etc.
The metric system is much simpler and more easily used.

Answer 2

as well as easier to change from one unit to another.