The aqueducts allow people in the city of Rome to get fresh water.
Work in the City
Grain was taken from farmers as a tax, and used to feed the poor in the city.
Taxing of Farmers
The Romans had almost every other day a holiday, so they didn't work half the year. They watched gladiator fights and chariot racing to keep themselves occupied.
Surpluses
Since they had every other day off, barely any work was finished. The city increasingly relied on other provinces for food because Rome could not produce enough.
Wasn't it a holiday just yesterday?
No work! Yay!
The farmers' surpluses were taken in the form of a tax, so that people in the city would not go hungry.
Give me your surplus-Emperor
The Empire needs your surplus.
The taken surplus would have been sold by the farmer, so the farmer would be angry about a government basically forcing farmers into poverty, and would see no point of growing a surplus.
Why should I even grow a surplus if it will just get taken away?