The fin helps give bow lift. It depends on your boat. Some boat's designs have good bow lift built in. Some bow lift; ie. cat's & tunnel's, is created with the help of the air flow around the hull.
Outboard boat's play with the CG and move the engine back to create more bow lift.
The faster you go the more lift the stern, (prop, outdrive) will have and the more bow lift you will need to counter the stern lift to balance the boat.
The "fin" helps neutralize the effects of the prop& drive lift.
My boat would run fine at 60mph above that it created to much stern lift and the fin is needed.