question #1 Sahih al-Bukhari 1900, Book 30, Hadith 10 : What is the significance of sighting the crescent moon in determining the start and end of Ramadan?

I heard Allah’s Messenger (ﷺ) saying, “When you see the crescent (of the month of Ramadan), start fasting,
and when you see the crescent (of the month of Shawwal), stop fasting; and if the sky is overcast (and
you can’t see it) then regard the month of Ramadan as of 30 days.”