You get one of these with your camera. You might want an extra one.
The waterproof hatch cover protects the openings in your camera for the external cables and the memory card.
Your camera will function perfectly well without the hatch cover in place. But it needs the cover to make it waterproof and dustproof.
If you are a SCUBA diver or snorkeler and occasionally lose things, you might consider buying an extra hatch cover just in case you lose the one that comes with the camera. You can't take the camera in the water without it.
If you order it with a camera it will come postage free.
Here you see the yellow hatch cover in place on the camera. Its a small but important accessory if you are going to get your camera wet.