Why is having someone else do our work "farming it out"?

The expression "farming out"--having someone else do part or all
of your work--is connected to the place where you get up before
dawn, work yourself to the bone, plant seeds, pray for rain, and
then sometimes see your crops washed away when you get exactly
what you asked for. But the connection is not that the expression
came from the place, but rather that both come from the same
source.

In the Middle Ages, the word "farm" meant a tax or rent, not the
land where you keep cows and pigs. The actual collection of the
tax was subcontracted out to a person known as a tax farmer.
Eventually the property for which the tax or rent was paid came
to be called a farm. And farming something out came to mean
subcontracting--assigning or paying someone to do our work for
us.