What the text says. Jesus enters the temple and "overthrew the tables of the money-changers, and the seats of them that sold the doves" (Matthew 21:12, ASV).
Why the tables were there at all. They were there because of a tax and a currency rule, and both are documented. Exodus 30:13-14 sets a half-shekel offering for the sanctuary from every male twenty years old and upward; by the first century this was an annual payment, and the collectors put the question to Peter directly, "Doth not your teacher pay the half-shekel?" (Matthew 17:24, ASV). The Mishnah preserves the collection calendar: the shekels were announced on the first of Adar, and then, in its own words, "On the fifteenth of [Adar] they would set up tables [of money changers] in the provinces. On the twenty-fifth they set them up in the Temple." (Shekalim 1:1, 1:3). The tables were necessary because sanctuary payments were reckoned in a specific silver standard, the Mishnah specifies payments in the sacred shekel "in the standard of Tyrian maneh" (Bekhorot 8:7), while pilgrims arrived carrying the ordinary coin of wherever they had come from, much of it bearing imperial images. So the coin had to be changed before it could be given, and a fee was charged for changing it. Those are the tables in Matthew 21:12: not an intrusion on the temple's business, but a fixed part of it, standing where the pilgrim crowds were.
Sources: Matthew 21:12; Matthew 17:24-27; Exodus 30:13-14; Mishnah Shekalim 1:1 and 1:3, Bekhorot 8:7; Jewish Encyclopedia, "Weights and Measures".
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