Suppose one observes that 2.0 centimeter cube of some liquid has a mass of 10.0 grams, and wishes to estimate how much a cubic meter of that liquid would weigh. One could compute a quantity, commonly called "density", by dividing the mass (10.0 grams) by the volume (8.0 cubic centimeters) of a sample, and then estimate the mass of a certain volume (1,000,000 cubic centimeters) of that liquid by multiplying that volume by the computed density (1.25 grams/cubic centimeter), yielding a mass (1,250,000 grams).
If all parts of the liquid in both cubes have identical characteristics, the mass of the large cube should be precisely 125,000 times as large as the mass of the small cube. If, however, some parts of the liquid in the large cube are compressed more than the liquid in the small cube, the mass of liquid necessary to fill the large cube might be greater than predicted.
Note that the fact that average density of any sample of liquid is equal to the total mass mass divided by the total volume will hold whether or not the liquid is uniform, but knowledge of the average density of a sample will be most useful when dealing with uniform materials.
Ohm's Law effectively defines a unit called an "ohm" which describes a ratio between the voltage across something and the current flowing through it. As with "density", such a quantity may be extremely useful in some situations, and far less useful in others. Ohm's law can usefully predict how things will behave if resistance stays relatively constant, and is useful in systems that behave as though resistance is relatively constant, but tends to be less useful in systems where the apparent resistance is highly variable.