Melville Street monuments

Monday morning I noticed that Melville Street south of the railroad tracks has finally had the last layer of paving applied. So it was a bit surprising to see a machine drilling holes in the newly-paved street.
I could not figure out what it was that they were putting into the holes, so I asked. A worker told me that they were installing monuments, or surveyor markers.
Left unasked and unanswered was the question of why they could not do this before they paved the street.