"Show ground grid"?

Was is the indended use of “Show ground grid” in the map/picture view?
Also I don’t understand which spots are marked in the “Show camera frames” option.

On the ground grid option, I’m not sure, but it is interesting to see how the 3D reconstruction “detected” the ground. And also for debug I guess.

With the “Show camera frames” option, you see where the camera was located. Position and orientation.

The ground grid shows an estimation of where the ground is as @chivos mentioned. It can for example be used to get a feeling for how well a road has been reconstructed.

Regarding the camera frames - these are the positions where other photos in front of the current one where taken. Clicking a camera frame starts a navigation to that photo.

In the regular viewer these two features are currently optional additional information that can be used if you like to dig a little deeper into the 3D reconstruction and get a feeling for the photo density in the surroundings.