$n = \dfrac{m}{M}$ answers three different exam questions depending on which letter is missing. Solve it for the mass and you get $m = n\,M$ — the recipe question, how much do I weigh out? Solve it for the molar mass and you get $M = \dfrac{m}{n}$ — the detective question, what IS this stuff?

That last one deserves its own paragraph, because it is how chemistry identifies an unknown. A molar mass is a fingerprint: water is 18.0 g/mol, carbon dioxide is 44.0, table salt is 58.5, glucose is 180.0. Weigh a sample, find out how many moles it holds, divide, and the number that falls out names the substance. Nothing else in a stockroom is quite that satisfying — a bottle with no label walks in, and arithmetic walks it back out with a name.