Bananas are rich in potassium. When eaten, you may specify to keep the banana peel. You also gain +1 DEX. I don't know why. Maybe because bananas are a pretty mobile fruit?
Bananas are one of the more lethal fruits to equip (not that you should equip them); they are bettered by the durian's spiky shell.
Monkeys are also attracted to bananas.
When planted, they become a banana tree after 17 turns of growth.