Rome needs tax money. The government needs to pay the legions, build roads, sewers, aqueducts, arenas, and pay for the welfare program in Rome. To get this tax money, Rome uses tax farmers. Tax farmers are Romans who pay a flat fee to the Roman Republic for the privilege of collecting taxes from a territory. To recoup these monies, tax farmers then levy a tax against every citizen in their territory.
Tax collectors expect to make a profit, as they are in the business of tax collection.
This is understood. However, under this system, there are many abuses, as the government can not control how each tax farmer runs their individual business. Under the Republic, elected officials use their positions to get rich. To get elected, some people are buying votes. The poor are quite happy to sell their votes to the highest bidder.