Why does Cessna Caravan amphibian have vertical fins on its horizontal stabilizers?

In simple terms, to be stable in yaw, a plane must have more vertical area behind the center of gravity than ahead of if. Side area ahead of the center of gravity decreases stability in yaw.

Look at how much the floats increase the profile area ahead of the wing - to compensate, they have to add vertical area to the tail,

The centroid of the area when viewed from the side must be behind the center of mass to prevent an aircraft from flying like a Frisbee.

The addition of floats moves the center of pressure forward, so additional area is added aft to compensate.

A larger vertical stab would do the same thing, but having the small fins allows them to be optional along with the floats.

It gives the airplane more stabilizer/rudder authority to compensate for the drag of the pontoons.