Useful scientific observations
By observing the behavior of the star just before its occultation, one can determine the nature of the atmosphere of the nearest object. If there is an atmosphere (Venus), the disappearance is gradual. If the star disappears suddenly, there is no atmosphere (Moon).
It is also possible to determine the diameter of the distant star by recording the gradual disapearance of the star.
The exact times of contact are important to determine the profile of the nearest object (moon, asteroid). And this in a greater resolution than with the VLT and other 8 meters telescopes.
Even the rings of the planet Uranus, which were only discovered in 1977 when the star SAO 142857 was occulted by the rings and its brightness reduced several times.
Good occultation observation requires that the observer has the knowledge of: 1. His GPS or Differential GPS position (check the accuracy using 3D maps). 2. The exact times of events. 3. His observation error and reaction time delay (usually about 0.4 seconds).