I recently went white water rafting with my GoPro HD Hero Naked for the first time, and I bought this as cheap insurance in case the tour outfit's pre-installed mounts happened to come loose from my helmet or I smacked heads with someone in the boat and knocked it off. After having a friend lose her GoPro in a lake when its suction-mount failed, I wasn't about to risk it.
There are several 3rd party floats for the GoPro cameras, but I liked this one for three reasons: 1) First and foremost, it comes with a spare waterproof backdoor. That means that once you install the floaty back, you aren't stuck with a float always being on your original waterproof backdoor from there on out. This was important to me since I like to use the waterproof backdoor to cut down on wind noise when using the GoPro on my motorcycle, and I didn't want the extra bulky look of the float sticking out when mounting the camera on my helmet. 2) It's a high-visibility orange. I saw the slightly cheaper ProFloaty Removable - GoPro Floaty Backdoor Accessory that's available in a bright green color, and the orange seemed like it would be visible in a greater variety of situations. 3) And finally, while not important in the grand scheme of things, having an OEM accessory like this just looks nice. I like having the logo and the "Be a hero" tagline on the back of the float; it just makes the whole kit look a bit more high-quality.
If you're going to be heading out into a body of water with your GoPro, do yourself a favor and spend the $15 to protect your $200 to $400 investment. You'll regret needing the insurance of a float and not having it a lot more than having the insurance and not needing it.