Farm is best used if your opponent is waiting over multiple turns, so you can profit enough Gold to afford expensive cards and combos. It becomes less relevant after Round 10, which gives +10 Gold per round, but may still be worth keeping if you spend Gold often.
Farm breaks even after 2 rounds since it was placed, assuming the Farm was not removed.
Some players may keep their Banana Farms in order to fund lots of strong bloons and/or afford multiple Ultra Rares. Additionally, strategies that heavily rely on Jungle's Bounty may have Farms to assist enough life regeneration to offset heavy HP losses.