I have just watched an interesting video about Moonphase watch. But why would I need to know about current Moon phase anyway as a layperson? I just can't think of any practical application for the moonphase indicator on the watch.
If you are Chinese, this can help you a lot. In the Chinese calendar, the new moon takes place on the first of the month. The full moon happens around 14th of the month. Basically, the month is the period between two consecutive new moons. This differs from the Gregorian calendar, where the month is only approximately correlated with the new moon. Since the period between two consecutive new moons is an integer multiple of the day and is not even constant, some months are shorter (小月), but some are longer (大月).
It is also worth noting that 12 times the length of the period between the two same phases (around 354 days) is approximately the same as one solar year (365.25 days), but not quite. Thus, they need to add leap month in some years.
Also, you can determine the angular distance between the Sun and the Moon. Thus, you can predict where the Moon is currently, which helps with tides or just observing the Moon. Many people which are not versed in astronomy think that the Moon is only visible in the night. With the moon phase you can determine whether the Moon is visible at the moment. Even in the daytime!
On 2nd thought this is actually a valid question: What does a layperson care about moon phases if (s)he is not into astrology, but interested in tangible applications? So far, I can imagine three possible applications:
- For sailors who want to navigate, the actual position of the moon on the sky is probably more interesting then its phase.
- For tidal prediction purposes, one would actually care more about the time of low and high tide on a daily basis, rather than only spring or neap tides.
- To decide for or against astronomical observation is a given night, it is better to be near new moon days, but cloudiness plays an important role as well.