The metric system was designed expressly for the internatioal trading
of commercial quantities; and as such 'cut to the quiick' with a
physical artifact that could be directly compared with real quantities
for that purpose. The metric system is not designed to show how
quantities of matter are comprised so that a net force (f), divided by
the acceleration (a) that it causes is a Constant; that is equivalent
to the ratio of a body's weight (w), divided by the acceleration at
which it will free fall.
The customary system always did, and still does that, but remains
historically encumbered with a lot of unessary and extraneous baggage.