question #3 Sahih Muslim 1081d, Book 13, Hadith 23 : Why is it important to count thirty days when the new moon is not sighted?

Observe fast when you see it (the new moon) and break fast when you see it (the new moon of Shawwal), but when (the actual position of the month is) concealed from you (on account of cloudy sky), then count thirty days.