I can think of 3 things that could entice someone to have different fins for freediving and spearfishing. Stiffness, maneuverability, and surface swimming.
Stiffness: Optimal stiffness for cb would be for deeper water than spearfishing, thus requiring more stiffness. Even for spearfishing many people will use different stiffness of fins depending on the depth they are spearing. A fin good for spearing at 25 meters may be really tiring if you’re spending all day spearing in 10 meter deep water.
Likewise a diver may use a soft fins for dynamics because the soft fins maybe more O2 efficient in the shallow water of a pool. They may chose stiffer fins for better acceleration when spearfishing.
Maneuverability: Some people find that their C4’s with side rails are not maneuverable enough for spearfishing where they need to be able to change direction easily. Shorter fins with no side rails may work better, depending on diving style and depth.
Surface swimming: When spearfishing from shore, the ability of the fins to surface swim without fatiguing the diver maybe more important than underwater performance. After all if the diver is too exhausted from the swim to dive, it doesn’t really matter how good they perform under the water.
My thoughts,
don
