I use carbon fiber because it is stiffer than fiberglass. I've used it on my recessed coaming, foot brace,

1/8 inch plywood with carbon fiber on both sides doesn't flex nearly as much as it would with fiberglass.

In the cockpit area it resists flexing caused by the weight (us) supported in a small area. Farther along the hull away from the cockpit the weight and buoyancy are more spread out so it's not needed. If it's used under the deck around the coaming, then it would be to stiffen the coaming area. It would also stiffen the whole 'tube' of the hull/deck even only on one side.
I'd try to keep from using it on the hull because it's very dark. In the sun it also heats up, but I've never had an yeeouch! from my carbon fiber coaming.

It's also used for hardware bits, rudders.