Where several coinages circulated, exchanging money was a necessary commercial service. Travelers, merchants, and pilgrims might possess coins unsuitable for a particular payment or inconvenient for the transaction they needed to make.
Jerusalem attracted pilgrims from many regions, especially during festivals. Currency exchange was therefore particularly important there. Temple-related payments added another reason for exchanging coins.
A changer evaluated coins and exchanged one form of money for another, normally receiving a fee for the service. Knowledge of weights, silver quality, denominations, and exchange relationships was important.
Coin finds and hoards demonstrate the circulation of multiple currencies and therefore the practical need for exchange services, especially in major commercial and pilgrimage centers.
Jewish and Greco-Roman sources attest money changing, while the Gospel Temple narratives specifically mention tables belonging to money changers.
Money changing itself was a practical economic activity. The biblical issue in the Temple narratives concerns Jesus' action and the meaning of commerce within that sacred setting.
Mark 11:15-17 and John 2:13-17 describe Jesus overturning the tables of money changers in the Temple precincts.
Money changers performed a genuine exchange function in an economy containing multiple coinages, and the Gospels place them in the Jerusalem Temple environment.
The Gospel accounts do not state that every money changer cheated pilgrims. Scholarly explanations for Jesus' action differ, so routine claims of universal profiteering or fraudulent exchange should be avoided.
Understanding why money changers were needed prevents us from reducing the Temple incident to a simplistic story about currency exchange itself being dishonest.