Fairly trips off the tongue, doesn't it? While a milligramme is 1/1000 grammes. 180 mg/dL means you have 18 grammes/litre of glucose in your blood, about the same sugar load as half-a-dozen ripe cherries.
180mg/dl = 1.8 grams per litre,
NOT 18 grams per litre.
The reason mol/mmol are used is that properties that vary as the number of particles can be compared, rather than weight.
If, for example, you wish to swap equivalent quantities of sodium for potassium, you'll need 23 for sodium for 39 of potassium.