A leftover from long gone times
Before timing machines were developed the timing of a watch needed several days or even weeks. The measure was a reference clock. The second was the unit that had to be kept within limits. To achieve a reasonable conclusion about the rate, the reference clock and the movement to be timed had to be compared each 12 or 24 hours. If a correction was needed it was either done by adjusting the regulator or by adjusting the regulation screws on the balance.
The later ones have the advantage that they can be adjusted in defined and exact doses. The turning of the screws is easily controlled, which results in a precisely defineable change of rate (as if one regulates a pendulum clock by operating the pendulum screw). The only danger is that the poise of the balance is disturbed easily.
The regulation of a movement by using the regulator is trouble-free in this respect. But there is another problem: the precision of the regulator is not the best: You move it for 1/100 of a mm, and the rate of the watch is changed for several seconds/day. 1/100 of a mm is not adjustable by hand, at least not without long training. Here the precision adjustment enters the scene. Only such a device makes the adjustment of a regulator for 1/100mm - feasible, but it means much more technical complexity.
This was the past, but nowadays we have timing machines, which allow to ascertain the actual rate of a watch within seconds. Even the ‚too slow' or ‚too fast' of a movement is easily detected and and corrected via the regulator (what an achievement compared to the 24h time frame of the formerly described old method!). A precision adjustment seems to have become obsolet...
Even worse: the regulator's curb pins are not ideal to maintain a precise rate of the watch in the long run. It is not without reason that today many watches are constructed without regulator. Considering this one is tempted to ask why the "VA" features a precision regulator.
The answer to this question cannot be found at a rational, time-measuring-related level. Rather at a sphere where one tries to attribute life into an artifact. A precision regulator sheds a little bit of soul into a watch. It is a tie to the past.