Akey role of moneychangers was to
bankers from different countries learn about each other's forms of currency
citizens determine how much value was assigned to different kinds of coins
guilds establish standards of quality for their merchandise and services
merchants from different places exchange one type of coin for another

A key role of moneychangers was to help : Merchants from different places exchange one type of coin to another.
Back then, moneychanger usually exist in every harbour of every cities. Traders from foreign country came to money changers to acquire the local coin (which back then is a form of currency) to do their businesses in that country

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The great depression