There are a number of reasons why you would want to gain a farmland off of a farmland:
You're running the farmlands out (and... something, like this ends the game or draws a line in the sand so that the opponent CAN'T pile out some other pile)
Your opponent's strategy needs to be able to farmland something (usually another farmland) into a province to work, and you don't have this limitation - ergo you want to run farmlands out ASAP, because then they're floundering around doing nothing.
The game will end before you see these cards again, and you're just maximizing points.
Possibly silk road
In any of these scenarios, which are already pretty unlikely, it's even far more unlikely that you need to do this TWICE in the same turn - and have the money and buys to do so. But of course, edge cases, edge cases everywhere.