They are used to propel the fish through the water, and in some cases they are used to keep water moving through the gills!
This is not a Betta question so it belongs elsewhere.
fishs eddy
Gills
fins
They use there legs as paddles and swim through the water.
The fins of a vampire squid serve as its primary means of propulsion. A vampire squid 'flies' through the water by flapping its pair of small fins projecting from the lateral sides of the mantle.
Humans use the fins of sharks in certain soup dishes
NO
You use fins and a nose cone on a bottle rocket because the cone reduces the drag on the rocket, and the fins help stabilize the rocket.
They use their fins to power through the water
Surfboards can have either 2, 3, or 4 fins. Some riders use just a single fin. The majority of riders use three fins.
it does nothing