Spent Apollo stages and lunar modules were steered into the surface deliberately, so the seismometers already deployed would record an impact of exactly known mass, speed and place.

You cannot calibrate a seismic network without a bang you understand, and there are no lunar earthquakes on demand. Nine such impacts were arranged between 1969 and 1972, and the resulting ringing — sometimes for over an hour — is most of what is known about the Moon's interior structure. Several of the craters have since been found from orbit.