The substantial difference is in the illumination percentage of the side facing the Earth. In the new moon phase, this percentage is 0% and becomes 100% at Full moon, about 14 days later. Thanks to the regular repetition of these moments, since ancient times, the lunar phases have been the basis of the lunisolar calendar.
However, it was difficult to establish the exact moment of the new moon, therefore, many ancient civilizations started the lunar month with the appearance of the first crescent after sunset. However, this is a very simplified description since the lunar motion is actually very complex, and the perspective with which we see the near side constantly changes, getting to see almost 60% of the lunar surface thanks to the phenomenon called libration.