NASA's Perseverance Mars Rover

They might also be AprilTags which look almost identical to Aruco markers but in either case they are so called fiducial markers.

They are typically used for pose estimation, which is the process of determining the relative position and rotation of the marker compared to one or more cameras.

The drill head is on a relatively long arm which can cause the kinematics (determining the head position by adding up angle sensors in the joints) to become a little imprecise.

Attaching markers to the head allows you to determine its position relative to the main body which you can use to verify/correct/supplement the position given by the kinematics.

In the end you know your drill head position with higher precision and certainty, which I would imagine is important when you want to take drill samples or shoot lasers at a tiny target.

While they could be used for camera calibration, ~~camera intrinsics usually don't change and~~ you'd typically have a calibration target with a much denser pattern on it. With only 3 measurement points you'd need to take hundreds of different pictures to get enough data for a good calibration.