The crater Mosting A was chosen to be the "fundamental location" for the Moon's coordinate system (latitude and longitude). But what exactly is a "fundamental location"? What does it have to do with the establishment of a coordinate system? Is it just so that observers have a way of easily projecting a spherical grid on the moon? And why was Mosting A chosen for the job?
Every coordinate system needs one point defined as exact and unambiguous - the fundamental location which all other location measurements in the vicinity are relative to.
This can be the origin of the coordinate system (like 0° longitude and 0° latitude) - but it can also be any other point instead. You choose that (or one) point which you can revisit the easiest in a reproducable manner.
The given point in the center of the Mösting A crater previously was one of the fundamental points for the Moons reference frame as it was/is easy to identify: it is very round, has a sharp edge, thus the mid-point is easy to identify. There seems to be some original correspondance on the topic available from the late 19th century on ADS and discussions on improving the map accuracy prior to the moon landing which all refer to these properties as being the reason of choice.
Currently it's defined more accurately via the retroreflectors placed on the moon by the Apollo missions. As such the coordinate for that were calculated such that 0°/0° is at the center of the on average visible disk of the Moon and then the location for that retroreflector was defined to be the resulting coordinates. Without such fundamental point, different measurements might use (slightly) differently-defined coordinate systems and comparison of data would become difficult. The retroreflector on that location is the largest one, so the easiest to find.
It's basically similarily arbitrary that the 0° latitude on Earth pass through Greenwich observatory.