This requirement is purely arbitrary.

For a measure-system measures something inherent in nature; otherwise it has no connexion with nature at all.

And that something which is measured by a particular measure-system may have a special relation to the phenomenon whose law is being formulated.

For example the gravitational field due to a material object at rest in a certain time-system may be expected to exhibit in its formulation particular reference to spatial and temporal quantities of that time-system.

The field can of course be expressed in any measure-systems, but the particular reference will remain as the simple physical explanation.
NOTE: ON THE GREEK CONCEPT OF A POINT The preceding pages had been passed for press before I had the pleasure of seeing Sir T.L.Heath's _Euclid in Greek_[14].