Yes, the exact location of the marker was recorded when it was put in, the X, Y, and Z (elevation) coordinates. They use it as a control to measure from. You might see things like this, which are temporary measuring targets, periodically. They're really important for having a baseline to go from to figure out if things have shifted/moved over time.

Much of the metro’s miles of tunnels were constructed using cut & cover, digging down from the surface, pouring concrete and laying steel, then covering it back up. (Not the deep stations along the redline, those were bored into bedrock). Disrupt that much soil and there will be compaction that occurs years later, and that can damage infrastructure built above. So metro monitors surface elevations around tunnels for that problem.

My guess is because they’re for doing monitoring on/around wmata property. Probably used for some combination for new construction and or vibration/settlement monitoring.

Even when very far away from Metro entrances on the surface you are still standing above or near a station, tunnel, or other Metro structure. Metro stations are massive.