May also be there to allow use by other cameras, as they are only on the side by the lander, so they must be there for the lander cameras only, probably so they can use them as distance and rotation markers for any image with them in it. The cameras probably already have a set of correction factors available for the lenses back at NASA so they know just how the images will be distorted by the fisheye lens as they go off axis, and the markers are there to verify the correction factors in image processing.