1. All calculations give the standard units of the measuring system. For instance, if you calculate a force using the 2nd law (F=ma) then when you multiply mass in kg times acceleration in m/s^2 you are guaranteed to get Newtons (N) without any need for dimensional analysis.
2. Scientist around the world can communicate without having to deal with arcane measurement systems. We can then compare results of different person because we have the same scale of measurement.