why do some goldsmiths and pawnbrokers use troy ounces (31grams, or 1.09 ounces) instead of the Imperial ounce?
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Same reason US people use Imperial instead of Metric. Historical standards perpetuated by commonality and cost of change outweighing the benefits.
All precious metals, when sold by ounces, are sold in troy ounces. Its just the way it’s always been. Of course, you can buy precious metals by gram weight as well, and that is always the same no matter what you’re weighing.
As for why, there have been competing measurement methods over the centuries. Some people thought the Troy system, with twelve ounces per pound to be best, and others thought the Avoirdupois system, with sixteen ounces per pound, to be best. Troy came first and everyone was trading gold and stuff in Troy, and then later came Avoirdupois, but since a lot of money was already minted in Troy ounces, precious metals stayed in the Troy system. The pound weight is the same for both systems. Remember that until fairly recently, money was actual metal, and not fiat currency. So a country would only have as much money as they could mine gold and silver. Mint marks also weren’t so much an indication that the coin was money so much as they guaranteed weight (at the time of minting) and purity. You could purchase things for parts of a coin, and coins were known to be cut into pieces and weighed to settle debts. Of course, people eventually began to drift away from this money system and just accepted the coins at their minted value, but that’s all how it came about.
TL;DR – because money.
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